[Vala] bad configure for MSYS2 + MingW 64 on Win7 64 bits

[oyster <lepto python gmail com>]

bad configure for MSYS2 + MingW 64 on Win7 64 bits
On my MSYS2 + MingW 64, there exists actually /bin/pkg-config.exe, but
the configure tries to find /bin/pkg-config.sh
it will fail without doubt. I have to

```
export GLIB_CFLAGS="$(pkg-config --cflags glib-2.0)"
export GLIB_LIBS="$(pkg-config --libs glib-2.0)"

export GMODULE_CFLAGS="$(pkg-config --cflags gmodule-2.0)"
export GMODULE_LIBS="$(pkg-config --libs gmodule-2.0)"

export LIBGVC_CFLAGS="$(pkg-config --cflags libgvc)"
export LIBGVC_LIBS="$(pkg-config --libs libgvc)"

./configure
./make -j8
```

btw, vala lives in github.com now. but the strange thing is that I did
not find issues part to report bugs