Revise loader compiler warnings and errors

[charles-lunarg]

#677 Revealed that the loader failed to compile in a configuration that isn't in github actions or webhook CI. This is a deficiency in the loader's cmake build scripts that should be rectified.

Cody helpfully explained how to see the build that highlighted the error in #679. Advised to use the commands which invoked the compilers as a reference when revising the build command in the repo.

[charles-lunarg]

I have enabled /W4 level errors in MSVC, as -Wextra and -Wall were present for gcc/clang builds.

This should help to catch further errors.