diff --git a/scripts/mmake.in b/scripts/mmake.in
index 7e34184b7..1f14e1920 100644
--- a/scripts/mmake.in
+++ b/scripts/mmake.in
@@ -71,8 +71,8 @@ Targets:
 		dependency files, and executables.
 "
 
-MERCURY_STDLIB_DIR=${MERCURY_STDLIB_DIR=@LIBDIR@}
-MERCURY_CONFIG_DIR=${MERCURY_CONFIG_DIR=@CONFIG_LIBDIR@}
+MERCURY_CONFIG_DIR=${MERCURY_CONFIG_DIR-${MERCURY_STDLIB_DIR-@CONFIG_LIBDIR@}}
+MERCURY_STDLIB_DIR=${MERCURY_STDLIB_DIR-@LIBDIR@}
 MMAKE_MAKE=${MMAKE_MAKE=@GNU_MAKE@}
 MERCURY_DEFAULT_GRADE=${MERCURY_DEFAULT_GRADE=@DEFAULT_GRADE@}
 MKTEMP=@MKTEMP@