This moves a lot of the preprocessor flags for what compiler/platform
we're using into a libqb-common.h header inside the libqb/include
folder. This gets included at the top of every libqb .cpp file, and is
intended to be fairly small, providing only necessary things like
_WIN32_WINNT (which needs to be defined before including <windows.h> or
friends).
Mostly old build scripts and helper files that are now covered by the
Makefile.
A notable deletion is the glew dll and lib files. These are unnecessary
because we compile `glew.c` directly rather than link against the dll or
lib copies we have.
Fixed an issue where Apple's Clang is forcefully used instead of GCC for compiling QB64 applications. (Fixes issues for those using true GCC via Homebrew and not Clang that just has a symlink of GCC).
