Fix compilation for win32-g++ and C++11

gcc defines __STRICT_ANSI__ implicitly for -std=c++0x. The MinGW headers
however omit the declaration of common functions like putenv then.

Instead of working around this MinGW particularity on a case by case basis, rather
just use gnu++0x for QtCore.

Change-Id: Iefe4e7f77014a4f1d501b149e34f7049deb52fb9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/corelib.pro

@@ -11,6 +11,9 @@ DEFINES   += QT_NO_USING_NAMESPACE
 win32-msvc*|win32-icc:QMAKE_LFLAGS += /BASE:0x67000000
 irix-cc*:QMAKE_CXXFLAGS += -no_prelink -ptused
 
+# otherwise mingw headers do not declare common functions like putenv
+win32-g++*:QMAKE_CXXFLAGS_CXX11 = -std=gnu++0x
+
 load(qt_module)
 
 QMAKE_DOCS = $$PWD/doc/qtcore.qdocconf