Update lib definitions

[RobLoach]

@konsumer Right now the lib directory builds out a static build to ease integration. I tried to build a shared library, but raylib_shared cmake was complaining. Have you been able to build raylib_shared throug cmake to use as a shared library before?

[ 37%] Built target glfw
[ 55%] Built target raylib
[ 57%] Linking C shared library libraylib-libretro-shared.so
/usr/bin/ld: ../vendor/raylib/raylib/libraylib.a(rtextures.c.o): relocation R_X86_64_TPOFF32 against `stbi__g_failure_reason' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: ../vendor/raylib/raylib/libraylib.a(rcore.c.o): relocation R_X86_64_PC32 against symbol `GLAD_GL_VERSION_1_0' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
make[2]: *** [lib/CMakeFiles/raylib-libretro-shared.dir/build.make:180: lib/libraylib-libretro-shared.so] Error 1
make[1]: *** [CMakeFiles/Makefile2:291: lib/CMakeFiles/raylib-libretro-shared.dir/all] Error 2
make: *** [Makefile:152: all] Error 2