The bit about CMAKE_MSVC_RUNTIME_LIBRARY has lost all context.
Originally it was related to OVERRIDE_MSVCCRT. Removing it fully
is more clear.

Cleanup the configure instructions. Using newer CMake cli.

Remove bit about the CMake GUI being tested. Doesn't make sense to
have in the README.md

Remove section about GLSLANG_TESTS. This documentation is now
outdated.