Generate and install pkg-config pc file

After installation, compiling new programs is possible with
    $ cc game.c `pkg-config --static --libs --cflags raylib`
or
    $ cc game.c `pkg-config --libs --cflags raylib`
depending on configuration

Also adds following configuration options:
- WITH_PIC "Compile static library as position-independent code"
- STATIC_RAYLIB "Build raylib as a static library"
- MACOS_FATLIB  "Build fat library for both i386 and x86_64 on macOS"

raylib.pc.in

@@ -0,0 +1,13 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: raylib
+Description: Simple and easy-to-use library to learn videogames programming
+URL: https://github.com/raysan5/raylib
+Version: @PROJECT_VERSION@
+Libs: -L${libdir} -lraylib
+Libs.private:@PKG_CONFIG_LIBS_PRIVATE@
+Requires.private:
+Cflags: -I${includedir}