Fix PRE_TARGETDEPS for iOS projects

This variable was ignored for iOS projects, because the generated
Makefile includes xcodebuild.mk that defines its own default target.

Export PRE_TARGETDEPS to the Makefile before including xcodebuild.mk
and use it there for the dependencies of the generic build targets.

Fixes: QTBUG-41325
Change-Id: I5faa82e05570974b5a844ae95b0a012c3badc64a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

mkspecs/features/uikit/xcodebuild.mk

@@ -27,8 +27,8 @@ distclean: clean_all
 $(EXPORT_SUBTARGETS): % : %-build
 
 # Generic targets
-%_first: $(firstword $(call targets, %)) ;
-%_all: $(call targets, %) ;
+%_first: $(EXPORT_PRE_TARGETDEPS) $(firstword $(call targets, %)) ;
+%_all: $(EXPORT_PRE_TARGETDEPS) $(call targets, %) ;
 
 # Actions
 %-build: ACTION = build

mkspecs/features/uikit/xcodebuild.prf

@@ -29,6 +29,8 @@ cmd = "$$QMAKE_QMAKE $$system_quote($$_PRO_FILE_) -spec macx-xcode $$args"
 debug(1, "Generating Xcode project in $$OUT_PWD using '$$cmd'")
 system("$$QMAKE_CD $$system_quote($$OUT_PWD) && $$cmd")
 
+QMAKE_EXTRA_VARIABLES += PRE_TARGETDEPS
+
 # Subtargets
 
 for(build, BUILDS): \