FindGTK2: Do not add freetype includes if they are not found

On some older system they are not required

Modules/FindGTK2.cmake

@@ -481,7 +481,9 @@ function(_GTK2_ADD_TARGET_INCLUDE_DIRS _var)
 
     if(TARGET GTK2::${_basename})
         foreach(_include ${ARGN})
-            set_property(TARGET GTK2::${_basename} APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES "${_include}")
+            if(${_include})
+                set_property(TARGET GTK2::${_basename} APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES "${_include}")
+            endif()
         endforeach()
     endif()
 endfunction()
@@ -571,7 +573,9 @@ endif()
 #
 
 find_package(Freetype QUIET)
-list(APPEND GTK2_INCLUDE_DIRS ${FREETYPE_INCLUDE_DIR_ft2build} ${FREETYPE_INCLUDE_DIR_freetype2})
+if(${FREETYPE_INCLUDE_DIR_ft2build} AND ${FREETYPE_INCLUDE_DIR_freetype2})
+    list(APPEND GTK2_INCLUDE_DIRS ${FREETYPE_INCLUDE_DIR_ft2build} ${FREETYPE_INCLUDE_DIR_freetype2})
+endif()
 
 foreach(_GTK2_component ${GTK2_FIND_COMPONENTS})
     if(_GTK2_component STREQUAL "gtk")