Fixes #899, which made apparent three issues with how static libraries
are handled in raylib-config.cmake:

- XPREFIX is set before pkg_check_modules, which causes a duplicate
  STATIC in variables, e.g. PC_RAYLIB_STATIC_STATIC_.*
- raylib_static is searched for, but no library exists with that name
- raylib_LDFLAGS isn't populated properly

This commit fixes these.

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.