Cannot Open Libmpfr.So.4 After Update on Ubuntu 18.04

Building GCC 4.6 - libmpfr.so.4 - cannot open shared object file

Make sure your library is acutally in the directory given and not in some lib subdirectory. Use export LD_LIBRARY_PATH=/users/xxxx/code/gcc/mpfr-3.0.1-install-cyprus as you have already suggested ;-)

missing libmpfr.so.6 when using a cross-compiler

In Ubuntu, apt command (Advanced Packaging Tool) is used for performing such functions as installation of new software packages, upgrade of existing software packages, updating of the package list index, and even upgrading the entire Ubuntu system.

Try this command :-

sudo apt-get update
sudo apt install libmpfr6

This will install shared libraries: libmpfr.so.6 to your Ubuntu system.

Hope It Helps !

VIM can't be used after system upgrade to ubuntu18.04

If this is the default Vim installed via your package manager, that's not supposed to happen. Something's messed up, and I would attempt a complete purge and reinstall of the Vim packages.
If you've compiled Vim yourself (then it's typically installed as /usr/local/bin/vim, or under your home directory), you need to recompile it.

Vim typically isn't a static, standalone binary. Especially for the language interpreters (Perl, Python, Ruby), it links to them via shared libraries. You'll see some of this in the :version output.

On the positive side, this means that any fixes and enhancements automatically get into Vim as well. On the negative side, this means that if one of those dependent libraries get upgraded (as happens during a system upgrade) and the old version is lost, you have to recompile Vim, so that the new dependencies are incorporated into it. The also implies that it's not advisable to copy a self-compiled Vim to another Linux system (unless you're sure it's on the exactly same version), and compile on each system instead.

When installing wldc libtinfo.so.5: cannot open shared object file

We need to install libtinfo.so.5 - on Ubuntu it's available as

apt-get install libtinfo5

Linux error while loading shared libraries: cannot open shared object file: No such file or directory

Update

While what I write below is true as a general answer about shared libraries, I think the most frequent cause of these sorts of message is because you've installed a package, but not installed the -dev version of that package.

Well, it's not lying - there is no libpthread_rt.so.1 in that listing. You probably need to re-configure and re-build it so that it depends on the library you have, or install whatever provides libpthread_rt.so.1.

Generally, the numbers after the .so are version numbers, and you'll often find that they are symlinks to each other, so if you have version 1.1 of libfoo.so, you'll have a real file libfoo.so.1.0, and symlinks foo.so and foo.so.1 pointing to the libfoo.so.1.0. And if you install version 1.1 without removing the other one, you'll have a libfoo.so.1.1, and libfoo.so.1 and libfoo.so will now point to the new one, but any code that requires that exact version can use the libfoo.so.1.0 file. Code that just relies on the version 1 API, but doesn't care if it's 1.0 or 1.1 will specify libfoo.so.1. As orip pointed out in the comments, this is explained well at here.

In your case, you might get away with symlinking libpthread_rt.so.1 to libpthread_rt.so. No guarantees that it won't break your code and eat your TV dinners, though.