diff --git a/configure.ac b/configure.ac
index cbad0992..37bb18c0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,7 +8,8 @@ LIBBTRFS_MAJOR=0
 LIBBTRFS_MINOR=1
 LIBBTRFS_PATCHLEVEL=1
 
-CFLAGS="-g -O1"
+CFLAGS=${CFLAGS:-"-g -O1"}
+AC_SUBST([CFLAGS])
 
 AC_PREREQ([2.60])