pkg-config: Empty Requires.private on shared-only build

If user doesn't build the static library,
`pkg-config --static --libs raylib` should be equivalent to
`pkg-config --libs raylib`.

raylib.pc.in

@@ -8,6 +8,6 @@ Description: Simple and easy-to-use library to learn videogames programming
 URL: http://github.com/raysan5/raylib
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lraylib
-Libs.private:@PKG_CONFIG_LIBS_PRIVATE@
+Libs.private: @PKG_CONFIG_LIBS_PRIVATE@
 Requires.private:
 Cflags: -I${includedir}

src/CMakeLists.txt

@@ -137,6 +137,8 @@ if(${PLATFORM} MATCHES "PLATFORM_DESKTOP")
         PUBLIC ${GRAPHICS}
     )
 
+    set(PKG_CONFIG_LIBS_PRIVATE ${__PKG_CONFIG_LIBS_PRIVATE})
+
     if (WITH_PIC)
       set_property(TARGET ${RAYLIB} PROPERTY POSITION_INDEPENDENT_CODE ON)
     endif()

utils.cmake

@@ -74,8 +74,8 @@ foreach(L ${LIBS_PRIVATE})
 
   set(LASTDIR ${DIR})
 
-  set(PKG_CONFIG_LIBS_PRIVATE ${PKG_CONFIG_LIBS_PRIVATE} ${DIR_OPT} ${FILE_OPT})
+  set(__PKG_CONFIG_LIBS_PRIVATE ${__PKG_CONFIG_LIBS_PRIVATE} ${DIR_OPT} ${FILE_OPT})
-  string (REPLACE ";" " " PKG_CONFIG_LIBS_PRIVATE "${PKG_CONFIG_LIBS_PRIVATE}")
+  string (REPLACE ";" " " __PKG_CONFIG_LIBS_PRIVATE "${__PKG_CONFIG_LIBS_PRIVATE}")
 endforeach(L)
 
 
@@ -86,7 +86,7 @@ function(link_libraries_to_executable executable)
   if (TARGET raylib_shared)
     target_link_libraries(${executable} raylib_shared)
   else()
-    target_link_libraries(${executable} raylib ${PKG_CONFIG_LIBS_PRIVATE})
+    target_link_libraries(${executable} raylib ${__PKG_CONFIG_LIBS_PRIVATE})
   endif()
 endfunction()