Updates and fixes to build_configs

[Lzard]

Adds missing macro definitions to SDL_build_config.h.cmake, removes unused ones, changes unneeded @VAR@ replacements to 1s, and fixes a few little things.

Those are issues I came across while working with SDL_build_config.h.cmake for a personal project that uses CMake configuration headers, but not CMake itself.

[slouken]

@madebr, can you take a look at this? It mostly looks good to me, I just want a second pair of eyes on this.

[slouken]

@madebr, ping?

[Lzard]

Rebased and fixed the issues:

• SDL_REVISION will indeed always be defined, so I dropped the related commit.
• HAVE_LIBUNWIND_H removed.
• calloc, free and realloc removed from symbols_to_check and _copysign and _fseeki64 added to it on Windows.

[slouken]

Looks good, thanks!