qmake: don't pass qmake configure arguments to sub-projects

the arguments after '--' are by definition meant only for the top-level
project, as that's where configure is invoked from. passing them to
sub-projects just adds noise to the make output and misleads users.

note that this specifically does not support qmake -r, which will break
if the subprojects rely on the arguments being absent. this isn't a
problem, because the qt build doesn't support qmake -r anyway.

note on cherry-picking: qt creator parses projects like qmake -r would,
so this is hypothetically more of a problem here. we presume that nobody
would actually include configure arguments in their build configuration,
which may or may not be true ...

Change-Id: I7ecff6212ce3137526005fc324a4a7ae45e3345e
(cherry picked from qtbase/34cc41d8a17e6e30f01f22c5d382c28d49ae37e1)
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/shared/proparser/qmakeglobals.cpp

@@ -135,7 +135,7 @@ QMakeGlobals::ArgumentReturn QMakeGlobals::addCommandLineArguments(
             if (arg.startsWith(QLatin1Char('-'))) {
                 if (arg == QLatin1String("--")) {
                     state.extraargs = args.mid(*pos + 1);
-                    *pos = args.size();
+                    args.erase(args.begin() + *pos, args.end());
                     return ArgumentsOk;
                 }
                 if (arg == QLatin1String("-after"))

src/shared/proparser/qmakeglobals.h

@@ -101,7 +101,7 @@ public:
     QProcessEnvironment environment;
 #endif
     QString qmake_abslocation;
-    QStringList qmake_args;
+    QStringList qmake_args, qmake_extra_args;
 
     QString qtconf;
     QString qmakespec, xqmakespec;