diff --git a/m4/ltp-warn_oldstyle.m4 b/m4/ltp-warn_oldstyle.m4
index 717e0a3..e6aedd7 100644
--- a/m4/ltp-warn_oldstyle.m4
+++ b/m4/ltp-warn_oldstyle.m4
@@ -23,7 +23,7 @@
 AC_MSG_CHECKING([if $CC supports $wflag])
 
 backup_cflags="$CFLAGS"
-CFLAGS+=" $wflag"
+CFLAGS="$CFLAGS $wflag"
 
 AC_LINK_IFELSE(
 	[AC_LANG_PROGRAM([])],