diff --git a/configure.ac b/configure.ac
index 6d83d64..59f1903 100644
--- a/configure.ac
+++ b/configure.ac
@@ -62,7 +62,7 @@ esac],[debug=false])
 if test x$debug = xtrue ; then
     DEBUG=1
     CXXFLAGS=""
-    CFLAGS="-O0 -ggdb -g3 ${werror_flag} -D_GNU_SOURCE=1 -Wall -pedantic -std=c99"
+    CFLAGS="-O0 -ggdb -g3 ${werror_flag} -D_GNU_SOURCE=1 -Wall -pedantic -std=c99 ${CFLAGS}"
 else
     DEBUG=0
     CXXFLAGS=""