ac_arg_enable invalid option

[John Ling]

Hello, I am currently using ac_arg_enable to define several 
--enable-feature options.  However, if a user were to specify an 
--enable-feature option that was invalid (that I did not define in my 
configure.ac).  How can I detect this and stop the configure process 
from continuing.

So for example, if a user specifies --enable-blah (where I never defined 
such an option), the configure script still continues to run to 
completion.  But I don't want it configure anything at all if there is a 
user typo.  How do I detect this invalid --enable option and just halt 
the configure script?

Thanks,
John