win32: fix build on Fedora >26

Default QMake configuration in mingw packages has changed since
Fedora 26 and some modifications are required in order to make the
ring client able to compile on it:
 - Change QMake spec from "win32-g++" to "mingw-w64-g++" which links
   with libqt5main.dll instead of the wrong libqtmain.dll.

With this patch, the client shouldn't be able to compile anymore on
Fedora <26. The corresponding patch in "client-windows" is also
required as parts of the fix include modifications of the project
file.

Change-Id: I7c8c6ac028c1bccc824691e8974a93569fb6bb9c
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

scripts/win_compile.sh

@@ -74,6 +74,6 @@ cd ../..
 fi
 mkdir -p build${ARCH}
 cd build${ARCH}
-${HOST}-qmake-qt5 ../RingWinClient.pro -r -spec win32-g++ RING=$INSTALL_PREFIX
+${HOST}-qmake-qt5 ../RingWinClient.pro -r -spec mingw-w64-g++ RING=$INSTALL_PREFIX
 make -j$CORES || exit 1
 make install