[MSYS2] fontconfig detection fails

Hi, this is the first of a series of reports aiming to build Qt on MSYS2 without extra patches.
As you probably already know, MSYS2 is a Windows build system, but with Linux-like features that might confuse the Qt build system.

After cloning the current 5.8 GIT cources, if I invoke configure like this:

./configure -release -opensource -confirm-license -platform win32-g++ -prefix /opt/qt5-git -opengl desktop -pkg-config -system-freetype -fontconfig

I get an error at the end that says:

ERROR: Feature 'fontconfig' was enabled, but the pre-condition '!config.win32 && !config.darwin && features.system-freetype && libs.fontconfig' failed.

The issue is in qtbase/src/gui/configure.json where fontconfig is considered to be a linux package only.
Instead, on MSYS2 (and on macOS macports/homebrew), fontconfig could be present and can be detected via pkg-config

Allowing Windows to use fontconfig has reflections also on qtbase/src/platformsupport/fontdatabases/fontdatabases.pro and on qtbase/src/plugins/platforms/minimal/qminimalintegration.cpp.
Patch attached