Print a more descriptive warning when a build is skipped.

This should help users understand which prerequisites are
missing from their system and may also help find invalid CI
configuration quicker.

Change-Id: If564825c11782b6e760f016893010008d9a551fa
Reviewed-by: Andras Becsi <andras.becsi@digia.com>

tools/qmake/mkspecs/features/functions.prf

 defineTest(isPlatformSupported) {
   static: return(false)
-  osx:lessThan(QMAKE_XCODE_VERSION, 5.1): return(false)
+  osx:lessThan(QMAKE_XCODE_VERSION, 5.1) {
+    warning("Using xcode version $$QMAKE_XCODE_VERSION, but at least version 5.1 is required to build Qt WebEngine.")
+    return(false)
+  }
 
   linux-g++*:!isGCCVersionSupported():return(false)
   !isPythonVersionSupported(): return(false)
   linux-g++*|win32-msvc2013|macx-clang: return(true)
   boot2qt: return(true)
+
+  warning("Qt WebEngine can currently only be built for Linux (GCC), Windows (MSVC 2013), OS X (XCode 5.1+) or Qt for Device Creation".)
   return(false)
 }
 
@@ -13,6 +18,7 @@ defineTest(isPythonVersionSupported) {
   python_major_version = $$system('python -c "import sys; print sys.version_info.major"')
   python_minor_version = $$system('python -c "import sys; print sys.version_info.minor"')
   greaterThan(python_major_version, 1): greaterThan(python_minor_version, 6): return(true)
+  warning("Using Python version "$$python_major_version"."$$python_minor_version", but at least Python version 2.7 is required to build Qt WebEngine.")
   return(false)
 }
 
@@ -20,6 +26,7 @@ defineTest(isGCCVersionSupported) {
   # The below will work for gcc 4.6 and up and also match gcc 5
   greaterThan(QT_GCC_MINOR_VERSION, 5):return(true)
   greaterThan(QT_GCC_MAJOR_VERSION, 4):return(true)
+  warning("Using gcc version "$$QT_GCC_MAJOR_VERSION"."$$QT_GCC_MINOR_VERSION", but at least gcc version 4.6 is required to build Qt WebEngine.")
   return(false)
 }