--- a/sci-mathematics/z3/z3-4.3.1.ebuild	
+++ a/sci-mathematics/z3/z3-4.3.1.ebuild	
@@ -53,7 +53,7 @@ src_configure() {
 	econf \
 		--host="" \
 		--with-python="${PYTHON}" \
-		$(use_with gmp) \
+		$(usex gmp "--with-gmp" "") \
 		SLIBFLAGS=" -Wl,-soname,lib${PN}.so.0.1 "
 	${EPYTHON} scripts/mk_make.py || die
 }