--- a/m4/warnings.m4
+++ b/m4/warnings.m4
@@ -14,24 +14,29 @@
 [m4_define([gl_AS_VAR_APPEND],
 [AS_VAR_SET([$1], [AS_VAR_GET([$1])$2])])])
 
+
 # gl_WARN_ADD(PARAMETER, [VARIABLE = WARN_CFLAGS])
 # ------------------------------------------------
 # Adds parameter to WARN_CFLAGS if the compiler supports it.  For example,
 # gl_WARN_ADD([-Wparentheses]).
+#
+# If VARIABLE is a variable name, AC_SUBST it.
 AC_DEFUN([gl_WARN_ADD],
 dnl FIXME: gl_Warn must be used unquoted until we can assume
 dnl autoconf 2.64 or newer.
 [AS_VAR_PUSHDEF([gl_Warn], [gl_cv_warn_[]_AC_LANG_ABBREV[]_$1])dnl
+AS_VAR_PUSHDEF([gl_Flags], [_AC_LANG_PREFIX[]FLAGS])dnl
 AC_CACHE_CHECK([whether _AC_LANG compiler handles $1], m4_defn([gl_Warn]), [
-  gl_save_compiler_FLAGS="$_AC_LANG_PREFIX[]FLAGS"
-  CPPFLAGS="${CPPFLAGS} $1"
+  gl_save_compiler_FLAGS="$gl_Flags"
+  gl_Flags="$gl_Flags $1"
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([])],
                     [AS_VAR_SET(gl_Warn, [yes])],
                     [AS_VAR_SET(gl_Warn, [no])])
-  _AC_LANG_PREFIX[]FLAGS="$gl_save_compiler_FLAGS"
+  gl_Flags="$gl_save_compiler_FLAGS"
 ])
 AS_VAR_IF(gl_Warn, [yes],
   [gl_AS_VAR_APPEND(m4_if([$2], [], [[WARN_CFLAGS]], [[$2]]), [" $1"])])
+AS_VAR_POPDEF([gl_Flags])dnl
 AS_VAR_POPDEF([gl_Warn])dnl
 m4_ifval([$2], [AS_LITERAL_IF([$2], [AC_SUBST([$2])], [])])dnl
 ])