macOS: Fix build issue with Qt 5.6

std::future is disabled when compiling with target 10.7, so
we need to enforce 10.8 when building against Qt 5.6 which has
default deployment target 10.7

Change-Id: I8d7b2646d0fb5268f82f5e01dae1f7e40a80bb8b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

qtcreator.pri

@@ -58,6 +58,12 @@ defineReplace(stripSrcDir) {
     return($$relative_path($$absolute_path($$1, $$OUT_PWD), $$_PRO_FILE_PWD_))
 }
 
+macos:!minQtVersion(5, 7, 0) {
+    # Qt 5.6 still sets deployment target 10.7, which does not work
+    # with all C++11/14 features (e.g. std::future)
+    QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.8
+}
+
 QTC_BUILD_TESTS = $$(QTC_BUILD_TESTS)
 !isEmpty(QTC_BUILD_TESTS):TEST = $$QTC_BUILD_TESTS