configure.in: fix messages

* configure.in (RUBY_FUNC_ATTRIBUTE): show the attribute name in
  the message if the macro name is not given.  fix up r45608.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46222 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -1526,7 +1526,7 @@ m4_ifval([$3], dnl
 )dnl
 m4_pushdef([attrib_code],[m4_bpatsubst([$1],["],[\\"])])
 m4_ifval([$4], [rbcv_cond=["$4"]; test "$rbcv_cond" || unset rbcv_cond])
-AC_CACHE_CHECK(for [$2] function attribute, rbcv,
+AC_CACHE_CHECK(for m4_ifval([$2],[$2],[$1]) function attribute, rbcv,
 [rbcv=x
 RUBY_WERROR_FLAG([
 for mac in \