79c93,95
< !win32: QMAKE_CXXFLAGS += $$QMAKE_CFLAGS_ISYSTEM $$PWD/../Third/
---
> !isEmpty(QMAKE_CFLAGS_ISYSTEM) {
>     QMAKE_CXXFLAGS += $$QMAKE_CFLAGS_ISYSTEM $$PWD/../Third/
> }