Work around std::wstring not being defined on Android.

libstdc++ requires wchar.h (which bionic provides), but it also requires
additional functionality which bionic does not provide, thus, std::wstring
proper basically doesn't exist.

Provide a compatibility shim so we can keep API intact. This feature existed in
a similar guise in the past, but was removed in
d868c9945a.

Change-Id: I6cab6f41d04ad9dde97e3ce73506f9d8a42043fb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstring.h

@@ -49,6 +49,15 @@
 
 #include <string>
 
+#if defined(Q_OS_LINUX_ANDROID)
+// std::wstring is disabled on android's glibc, as bionic lacks certain features
+// that libstdc++ checks for (like mbcslen).
+namespace std
+{
+    typedef basic_string<wchar_t> wstring;
+}
+#endif
+
 #include <stdarg.h>
 
 #ifdef truncate