How to Get Cmake to Use the Default Compiler on System Path

How to specify new GCC path for CMake

Do not overwrite CMAKE_C_COMPILER, but export CC (and CXX) before calling cmake:

export CC=/usr/local/bin/gcc
export CXX=/usr/local/bin/g++
cmake /path/to/your/project
make

The export only needs to be done once, the first time you configure the project, then those values will be read from the CMake cache.

UPDATE: longer explanation on why not overriding CMAKE_C(XX)_COMPILER after Jake's comment

I recommend against overriding the CMAKE_C(XX)_COMPILER value for two main reasons: because it won't play well with CMake's cache and because it breaks compiler checks and tooling detection.

When using the set command, you have three options:

without cache, to create a normal variable
with cache, to create a cached variable
force cache, to always force the cache value when configuring

Let's see what happens for the three possible calls to set:

Without cache

set(CMAKE_C_COMPILER /usr/bin/clang)
set(CMAKE_CXX_COMPILER /usr/bin/clang++)

When doing this, you create a "normal" variable CMAKE_C(XX)_COMPILER that hides the cache variable of the same name. That means your compiler is now hard-coded in your build script and you cannot give it a custom value. This will be a problem if you have multiple build environments with different compilers. You could just update your script each time you want to use a different compiler, but that removes the value of using CMake in the first place.

Ok, then, let's update the cache...

With cache

set(CMAKE_C_COMPILER /usr/bin/clang CACHE PATH "")
set(CMAKE_CXX_COMPILER /usr/bin/clang++ CACHE PATH "")

This version will just "not work". The CMAKE_C(XX)_COMPILER variable is already in the cache, so it won't get updated unless you force it.

Ah... let's use the force, then...

Force cache

set(CMAKE_C_COMPILER /usr/bin/clang CACHE PATH "" FORCE)
set(CMAKE_CXX_COMPILER /usr/bin/clang++ CACHE PATH "" FORCE)

This is almost the same as the "normal" variable version, the only difference is your value will be set in the cache, so users can see it. But any change will be overwritten by the set command.

Breaking compiler checks and tooling

Early in the configuration process, CMake performs checks on the compiler: Does it work? Is it able to produce executables? etc. It also uses the compiler to detect related tools, like ar and ranlib. When you override the compiler value in a script, it's "too late", all checks and detections are already done.

For instance, on my machine with gcc as default compiler, when using the set command to /usr/bin/clang, ar is set to /usr/bin/gcc-ar-7. When using an export before running CMake it is set to /usr/lib/llvm-3.8/bin/llvm-ar.

How to get CMake to use the default compiler on system PATH?

As is already written in the answer to the other question, CMake prefers the generic compiler names cc and c++ when searching for the C and C++ compilers. These probably refer to GNU version 4.1 compilers on your system.

Anyway, to force CMake to use the default compilers on the system path, add the following code to the beginning of your outermost CMakeLists.txt.

find_program(CMAKE_C_COMPILER NAMES $ENV{CC} gcc PATHS ENV PATH NO_DEFAULT_PATH)
find_program(CMAKE_CXX_COMPILER NAMES $ENV{CXX} g++ PATHS ENV PATH NO_DEFAULT_PATH)
...
project (Foo C CXX)

The find_program calls must occur before the call to project or enable_language.

How to Get CMake to Use Default Compiler on System PATH?

CMake honors the setting of the PATH environment variable, but gives preference to the generic compiler names cc and c++. To determine which C compiler will be used by default under UNIX by CMake, run:

$ which cc

To determine the default C++ compiler, run:

$ which c++

If you generate a symbolic link c++ in /opt/gcc-4.8/bin which points to /opt/gcc-4.8/bin/g++, CMake should use GCC 4.8 by default.

How to find compiler path automatically in cmake?

Variables Intel_C_COMPILER and Intel_CXX_COMPILER have trailing newline. Way for removing that newline are described in that question and its answers: How to strip trailing whitespace in CMake variable?

E.g., you may run execute_process with OUTPUT_STRIP_TRAILING_WHITESPACE option, so it will behave similar to the shell's backtick operator (`which icc`).

Detailed description

Most of shell utilities output single- (or even multi-) line information with trailing newline. And utility which is not an exception. With trailing newline an output looks nice when one run these utilities in the terminal.

But when run such utility in the script and grab its output programmatically, one need to care about such newline.

Setting default compiler in CMake

With CMake version 3.15 or later, you can set the CMAKE_GENERATOR environment variable to specify the default generator to be used on your system.

How to specify a compiler in CMake?

To select a specific compiler, you have several solutions, as exaplained in CMake wiki:

Method 1: use environment variables

For C and C++, set the CC and CXX environment variables. This method is not guaranteed to work for all generators. (Specifically, if you are trying to set Xcode's GCC_VERSION, this method confuses Xcode.)
For example:

CC=gcc-4.2 CXX=/usr/bin/g++-4.2 cmake -G "Your Generator" path/to/your/source

Method 2: use cmake -D

Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path on the command-line using cmake -D.
For example:

cmake -G "Your Generator" -D CMAKE_C_COMPILER=gcc-4.2 -D CMAKE_CXX_COMPILER=g++-4.2 path/to/your/source

Method 3 (avoid): use set()

Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path in a list file using set(). This must be done before any language is set (ie: before any project() or enable_language() command).
For example:

set(CMAKE_C_COMPILER "gcc-4.2")
set(CMAKE_CXX_COMPILER "/usr/bin/g++-4.2")

project("YourProjectName")

The wiki doesn't provide reason why 3rd method should be avoided...

CMake on Windows

Because CMake's error message is misleading here, I think it warrants a little more detailed answer.

In short, you ran into a chicken-and-egg kind of a problem.

CMake's compiler detection is mighty, but since - during the first try -

you didn't give any explicit generator to use with -G
it couldn't find a Visual Studio installed
it couldn't find any C/C++ compiler in your PATH environment
it couldn't find a CC environment variable defined with the full path to a compiler

It was defaulting to nmake.

Now here comes the problem: it does remember your implicit generator/compiler choice in it's variable cache (see CMAKE_GENERATOR in CMakeCache.txt). What is a very useful feature, if you have multiple compilers installed.

But if you then declare the CC environment variable - as the error message suggests - it's too late since your generator's choice was remembered in the first try.

I see two possible ways out of this:

Overrule the generator choice by given the right one with cmake.exe -G "MinGW Makefiles" .. (as the answer linked by @Guillaume suggests)
Delete your project's binary output directory (including CMakeCache.txt) and do cmake.exe .. after you added your compiler's bin folder to your PATH environment.

References

Running CMake on Windows
What is the default generator for CMake in Windows?
CMake error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found
CMake: how to specify the version of Visual C++ to work with?