Fix the Mac build with a -no-framework Qt

If QMAKE_FRAMEWORKPATH is empty, this would add an empty entry to the list
and would end up with a -F with no argument added to the clang command line.

This would fail the build since the next switch would be eaten by the -F.

Change-Id: I42657e781278fd56d6664a42fb4222eb83b3cd73
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>

tools/qmake/mkspecs/features/gyp_generator.prf

@@ -82,7 +82,7 @@ for (lib, LIBS): GYP_CONTENTS += "      '$$lib',"
 for (rpath, QMAKE_RPATHDIR): GYP_CONTENTS += "      '$$QMAKE_RPATH$$rpath',"
 GYP_CONTENTS += "    ],"
 
-macx {
+!isEmpty(QMAKE_FRAMEWORKPATH) {
     GYP_CONTENTS += "    'mac_framework_dirs': ["
     GYP_CONTENTS += "      '$$QMAKE_FRAMEWORKPATH',"
     GYP_CONTENTS += "    ],"