Add CONFIG += no_moc_predefs to allow one to escape problems

Sometimes, users need to add compiler flags to QMAKE_CXXFLAGS that
aren't supposed to be passed to the predefs dump. That's especially true
for -include options, as that would change completely what's
defined. Not to mention that -include is a preprocessor option and
shouldn't be in CXXFLAGS in the first place (Automake has CPPFLAGS, but
qmake only has INCLUDEPATH and DEFINES).

[ChangeLog][qmake] Added the ability to suppress the collection of the
compiler predefined macros for moc's use. To disable the collection, use
CONFIG += no_moc_predefs.

Task-number: QTBUG-58857
Change-Id: I4139d5f93dcb4b429ae9fffd14a34d49825d9b85
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/features/moc.prf

@@ -27,7 +27,7 @@ win32:count(MOC_INCLUDEPATH, 40, >) {
 # QNX's compiler sets "gcc" config, but does not support the -dM option;
 # UIKit builds are always multi-arch due to simulator_and_device (unless
 # -sdk is used) so this feature cannot possibly work.
-if(gcc|intel_icl|msvc):!rim_qcc:!uikit {
+if(gcc|intel_icl|msvc):!rim_qcc:!uikit:!no_moc_predefs {
     moc_predefs.name = "Generate moc_predefs.h"
     moc_predefs.CONFIG = no_link
     gcc:            moc_predefs.commands = $$QMAKE_CXX $$QMAKE_CXXFLAGS -dM -E -o ${QMAKE_FILE_OUT} ${QMAKE_FILE_IN}