make gbuild-to-ide VS files work with separate builddir

Change-Id: I847885885ed12dbbd64e3b694799e4835497d736
Reviewed-on: https://gerrit.libreoffice.org/11799
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>