diff --git a/generator/Makefile.am b/generator/Makefile.am
index 1c7565ac3..c690f7e0a 100644
--- a/generator/Makefile.am
+++ b/generator/Makefile.am
@@ -15,6 +15,9 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
+# Old RHEL 5 autoconf doesn't have abs_builddir.
+abs_builddir ?= $(shell pwd)
+
 # In alphabetical order.
 sources = \
 	actions.ml \