Patch for AC_FC_MODULE_OUTPUT_FLAG

[Thomas Jahns]

Hello,

I found AC_FC_MODULE_OUTPUT_FLAG does not know the -mdir option to nagfor and
added that. And while I was at it, I also did some change to make the shell code
a little safer. Can you include the patch as is or is it missing something?

Regards, Thomas

[Paul Eggert]

Thanks, I installed that, with the commit message redone to match GNU style.