diff --git a/gcc/gcc/configure.ac b/gcc/gcc/configure.ac
index 63a0be9974..8d0bc997be 100644
--- a/gcc/gcc/configure.ac
+++ b/gcc/gcc/configure.ac
@@ -1010,6 +1010,7 @@ case $build in
     build_math_library=
     ;;
 esac
+AC_SUBST(build_math_library)
 
 # These libraries may be used by collect2.
 # We may need a special search path to get them linked.