macOS: Detect changes to the platform SDK and ask the user to deal with it

Otherwise the SDK upgrade (or downgrade) may subtly and silently affect
the resulting binary, or if it results in build breaks, the user won't
know why.

We limit it to applications for now, as that's the point where it's
most important to catch the SDK upgrade, but technically we should
also do this for intermediate libraries. Doing it for everything
will likely incur a performance cost, so we skip that for now.

Change-Id: I8a0604aad8b1e9fba99848ab8ab031c07fd50dc4
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

mkspecs/features/mac/default_post.prf

@@ -1,5 +1,11 @@
 load(default_post)
 
+# Detect changes to the platform SDK. Apps only for now
+contains(TEMPLATE, .*app):!macx-xcode {
+    QMAKE_EXTRA_VARIABLES += QMAKE_MAC_SDK QMAKE_MAC_SDK_VERSION
+    QMAKE_EXTRA_INCLUDES += $$shell_quote($$PWD/sdk.mk)
+}
+
 !no_objective_c:CONFIG += objective_c
 
 qt {

mkspecs/features/mac/sdk.mk

@@ -0,0 +1,12 @@
+CURRENT_MAC_SDK_VERSION := $(shell /usr/bin/xcrun --sdk $(EXPORT_QMAKE_MAC_SDK) -show-sdk-version)
+
+ifneq ($(CURRENT_MAC_SDK_VERSION),$(EXPORT_QMAKE_MAC_SDK_VERSION))
+    $(info The platform SDK has been changed from version $(EXPORT_QMAKE_MAC_SDK_VERSION) to version $(CURRENT_MAC_SDK_VERSION).)
+    $(info This requires a fresh build. Please wipe the build directory completely,)
+    $(info including any .qmake.stash and .qmake.cache files generated by qmake.)
+    # FIXME: Ideally this should be advertised as just running make distclean, or we
+    # should even do it automatically by having proper makefile dependencies between
+    # .qmake.stash and the SDK version, but as qmake doesn't seem to be consistent in
+    # how it deals with .qmake.stash as a dependency we need to defer that until later.
+    $(error ^)
+endif