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.