[PATCH] Have configure complain about unknown options

Currently, configure ignores unknown --enable/disable/with/without
options. Personally I find this behaviour annoying, so here is a patch
that fixes it. Example:

$ ./configure --enable-depends
checking build system type... i686-pc-linux-gnu
checking host system type... i686-pc-linux-gnu
<snip>
checking whether to build with Bonjour support... no
checking whether to build with OpenSSL support... no
*** Unknown feature: enable_depends ***
$

Now, while the error message could do with some help (feedback welcome)
I personally find this to be quite useful.

This patch will detect any argument that isn't declared using one of
the PGAC_ARG macros. This means that if there are any other
--enable/with arguments we accept but don't declare, this will barf on
them too. Do we have any of those? If so, we should document them
somewhere, if necessary creating a PGAC_ARG_HIDDEN so they can pass
this test.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.