--- /dev/null
+dnl ----------------------------------------------------------------------
+dnl This whole bit snagged from libstdc++-v3.
+
+dnl
+dnl GCC_ENABLE
+dnl (FEATURE, DEFAULT, HELP-ARG, HELP-STRING)
+dnl (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, permit a|b|c)
+dnl (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, SHELL-CODE-HANDLER)
+dnl
+dnl See docs/html/17_intro/configury.html#enable for documentation.
+dnl
+AC_DEFUN([GCC_ENABLE],[dnl
+m4_define([_g_switch],[--enable-$1])dnl
+m4_define([_g_help],[AC_HELP_STRING(_g_switch$3,[$4 @<:@default=$2@:>@])])dnl
+ AC_ARG_ENABLE($1,_g_help,
+ m4_bmatch([$5],
+ [^permit ],
+ [[
+ case "$enableval" in
+ m4_bpatsubst([$5],[permit ])) ;;
+ *) AC_MSG_ERROR(Unknown argument to enable/disable $1) ;;
+ dnl Idea for future: generate a URL pointing to
+ dnl "onlinedocs/configopts.html#whatever"
+ esac
+ ]],
+ [^$],
+ [[
+ case "$enableval" in
+ yes|no) ;;
+ *) AC_MSG_ERROR(Argument to enable/disable $1 must be yes or no) ;;
+ esac
+ ]],
+ [[$5]]),
+ [enable_]m4_bpatsubst([$1],-,_)[=][$2])
+m4_undefine([_g_switch])dnl
+m4_undefine([_g_help])dnl
+])
+