Homebrew doesn't ship 32-bit libraries anymore, so using both
-DUSE_EXTERNAL_GLFW and -DMACOS_FATLIB with a Homebrew GLFW would fail.
With -DUSE_EXTERNAL_GLFW=OFF, this is not a problem, but I think keeping
it off by default makes more sense. If someone wants universal
libraries, they can just toggle it.

Remove that link_libraries_to_executable() hack and defines a proper
raylib target that can be used with target_link_libraries.

The same target is also available for external (user) code by using
find_package(raylib).

This results in:

- Remove hardcoded build directories from examples and games CMakeLists.txt
- Allow rlgl_standalone and other special examples to be built easily
- Allow CMake projects to find_package(raylib instead of fiddling with pkg-config
- Makes code a little more maintainable
- Fixes #471, #606.
- Makes code less confusing by removing the double use of PLATFORM (#584).

Note that this is still not _The Right Way_(TM), because normally
raylib-config.cmake (or its includes) would be automatically generated.
I didn't manage to get that to work though, so I went the easier route
of just wrapping pkg_check_modules for consumption by find_package.

also makes one configuration shared-only

dr_flac is not built on i386, because GCC 4.8 complains about asm("cpuid"
clobbering ebx, as it's using ebx for PIC.

Instead of downloading a newer GCC (and multilib),
disable FLAC for that one configuration...

as well as -DUSE_EXTERNAL_GLFW.

Nips issues like #584 in the bud.

Did this ever work? Surely, doesn't look like it...

Also added new compilation FLAGS for that pourpose

I would have liked config.h to be selected by include dir configuration,
but this way is less intrusive.