Display warning message when pkg_config was not found

Author: tobbi

mk/cmake/SuperTux/ProvideSDL2.cmake

@@ -33,8 +33,12 @@ else()
     else()
       # 64-bit
       include(FindPkgConfig)
-      pkg_search_module(SDL2 REQUIRED sdl2>=2.0.1)
-      pkg_search_module(SDL2IMAGE REQUIRED SDL2_image>=2.0.0)
+      if(NOT PKG_CONFIG_FOUND)
+        message(WARNING "Trying to find libraries using pkg_search_module, but pkg-config was not found. Install pkg-config to continue.")
+      else()
+        pkg_search_module(SDL2 REQUIRED sdl2>=2.0.1)
+        pkg_search_module(SDL2IMAGE REQUIRED SDL2_image>=2.0.0)
+      endif()
     endif()
   endif()
 endif()