Add Q_COMPILER_THREADSAFE_STATICS (a.k.a. "Magic Statics")

It had been so far only supported by GNU-style compilers due to the
IA-64 portable C++ ABI. But it's mandated by C++11, so let's add the
macro and use it in Q_GLOBAL_STATIC.

Looks like Visual Studio "14" will support it.

Change-Id: I9710b5146606c7e494c43413f49900419396cfe0
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: David Faure <david.faure@kdab.com>