--- a/m4/regex.m4
+++ b/m4/regex.m4
@@ -9,6 +9,23 @@
 		USE_REGEX=$withval,
 		USE_REGEX=yes)
 
-    test "$USE_REGEX" = "yes" && LIBOBJS="$LIBOBJS regex.o"
+    if test "$USE_REGEX" = yes; then
+
+      LIBOBJS="$LIBOBJS regex.o"
+
+    else
+      if test x = y; then
+	# This code is deliberately never run via ./configure.
+	# FIXME: this is a hack to make autoheader put the corresponding
+	# HAVE_* undef for this symbol in config.h.in.  This saves me the
+	# trouble of having to maintain the #undef in acconfig.h manually.
+	AC_CHECK_FUNCS(LIBC_REGEX)
+      fi
+      # Defining it this way (rather than via AC_DEFINE) short-circuits the
+      # autoheader check -- autoheader doesn't know it's already been taken
+      # care of by the hack above.
+      ac_kludge=HAVE_LIBC_REGEX
+      AC_DEFINE_UNQUOTED($ac_kludge)
+    fi
   ]
 )