diff --git a/gmp-6.0.0a.ebuild b/gmp-6.0.0a.ebuild
index c5bddb5..18f7fca 100644
--- a/gmp-6.0.0a.ebuild
+++ b/gmp-6.0.0a.ebuild
@@ -18,7 +18,7 @@ SRC_URI="mirror://gnu/${PN}/${MY_P}.tar.xz
 LICENSE="LGPL-3"
 SLOT="0"
 KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~s390 ~sh ~sparc ~x86 ~amd64-fbsd ~sparc-fbsd ~x86-fbsd"
-IUSE="doc cxx pgo static-libs"
+IUSE="doc cxx pgo static-libs userland_BSD"
 
 DEPEND="sys-devel/m4
 	app-arch/xz-utils"
@@ -64,6 +64,9 @@ multilib_src_configure() {
 	esac
 	export GMPABI
 
+	# force use gm4 on Gentoo/FreeBSD. bug 511680.
+	use userland_BSD && export gmp_cv_prog_m4=gm4
+
 	tc-export CC
 	ECONF_SOURCE="${S}" econf \
 		--localstatedir=/var/state/gmp \