qdoc: don't add clang inc dir in system location to INCLUDEPATH

qmake passes system paths via -isystem, which wreaks havoc.

Fix copied from QtCreator's build system.
See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129



Task-number: QTBUG-65920
Change-Id: Ifae6aea6ccb1a15282657377880a8f5a37ea42a0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/qdoc/qdoc.pro

@@ -12,7 +12,8 @@ qtHaveModule(qmldevtools-private) {
 }
 
 LIBS += $$CLANG_LIBS
-INCLUDEPATH += $$CLANG_INCLUDEPATH
+!contains(QMAKE_DEFAULT_INCDIRS, $$CLANG_INCLUDEPATH): INCLUDEPATH += $$CLANG_INCLUDEPATH
+
 !disable_external_rpath: QMAKE_RPATHDIR += $$CLANG_LIBDIR
 DEFINES += $$shell_quote(CLANG_RESOURCE_DIR=\"$${CLANG_LIBDIR}/clang/$${CLANG_VERSION}/include\")