Add some interesting warning options to the compilation

This should help improve the cleanliness of our source code, including
compliance with the C++ standards. They apply to all of our code except
examples (they don't load qt_common.prf).

Change-Id: Ia0aac2f09e9245339951ffff13c94663c1901766
Reviewed-by: David Faure <david.faure@kdab.com>

mkspecs/features/qt_common.prf

@@ -44,6 +44,21 @@ contains(TEMPLATE, .*lib) {
     QMAKE_PRL_INSTALL_REPLACE += lib_replace
 }
 
+# Extra warnings for Qt non-example code, to ensure cleanliness of the sources.
+# The block below may turn these warnings into errors for some Qt targets.
+# -Wdate-time: warn if we use __DATE__ or __TIME__ (we want to be able to reproduce the exact same binary)
+# -Wvla: use of variable-length arrays (an extension to C++)
+clang {
+    # Clang 3.5 introduced -Wdate-time
+    # The conditional assumes we aren't compiling against Clang 2.x anymore
+    greaterThan(QT_CLANG_MAJOR_VERSION, 3)|greaterThan(QT_CLANG_MINOR_VERSION, 4): \
+        QMAKE_CXXFLAGS_WARN_ON += -Wdate-time
+} else: gcc:!intel_icc {
+    QMAKE_CXXFLAGS_WARN_ON += -Wvla
+    # GCC 5 introduced -Wdate-time
+    greaterThan(QT_GCC_MAJOR_VERSION, 4): QMAKE_CXXFLAGS_WARN_ON += -Wdate-time
+}
+
 warnings_are_errors:warning_clean {
     # If the module declares that it has does its clean-up of warnings, enable -Werror.
     # This setting is compiler-dependent anyway because it depends on the version of the

mkspecs/features/qt_module_headers.prf

@@ -124,7 +124,8 @@ headersclean:!internal_module {
                 hcleanFLAGS -= -Wshadow
             }
         } else {
-            hcleanFLAGS += -Wchar-subscripts
+            # options accepted by GCC and Clang
+            hcleanFLAGS += -Wchar-subscripts -Wold-style-cast
 
             !contains(QT_ARCH, arm):!contains(QT_ARCH, mips): \
                 hcleanFLAGS += -Wcast-align