zip.m4: Fix --with/--without argument handling.
diff --git a/m4/zip.m4 b/m4/zip.m4
index 398d1be..19fa492 100644
--- a/m4/zip.m4
+++ b/m4/zip.m4
@@ -1,10 +1,11 @@
-dnl Test for either zlib or bzlib.
+dnl -*- Autoconf -*- test for either zlib or bzlib.
dnl Defines --with-$1lib argument, $2LIB automake conditional,
dnl and sets AC_DEFINE(USE_$2LIB) and LIBS.
-AC_DEFUN([eu_ZIPLIB], [with_[$1]lib=default
+AC_DEFUN([eu_ZIPLIB], [dnl
AC_ARG_WITH([[$1]lib],
-AC_HELP_STRING([--with-[$1]lib], [support g[$1]ip compression in libdwfl]))
+AC_HELP_STRING([--with-[$1]lib], [support g[$1]ip compression in libdwfl]),,
+ [with_[$1]lib=default])
if test $with_[$1]lib != no; then
AC_SEARCH_LIBS([$4], [$3], [with_[$1]lib=yes],
[test $with_[$1]lib = default ||