iOS: Create folder for SUBDIR projects

In case of a SUBDIR qmake project, it tried to cd into a directory
that did not exist yet. I needed to run qmake twice to get it working.
This fixes it.

Change-Id: I6d322e9a7c96a9d82df77b9ba5f19711a8180ed0
Task-number: QTBUG-37429
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

mkspecs/macx-ios-clang/features/default_post.prf

@@ -71,6 +71,7 @@ equals(TEMPLATE, app) {
                 distclean.depends = xcode_distclean
                 QMAKE_EXTRA_TARGETS += distclean
 
+                mkpath($$OUT_PWD)|error("Aborting.")
                 args =
                 for(arg, QMAKE_ARGS): \
                     args += $$system_quote($$arg)