Fix firebird without cygwin gcc

The firebuild buildsystem calls windres, which depends on cygwin gcc,
use rc.exe so that a windows build without cygwin gcc will succeed.

Change-Id: Ic7719749b3806232912e3eb8b1ede11e6eb3c10c
Reviewed-on: https://gerrit.libreoffice.org/11619
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>