Add DJGPP-specific binary paths to configure.in

[DJ Delorie]

> I don't see where configure.in assembler feature tests use assembler from
> user path. They look only for newly built gas and assemblers in that path
> list, where I want to add DJGPP paths.

$gcc_cv_as is set to just plain "as" way before the checks in specific
directories.  If it can't find any as where it's looking, it just runs
"as" and whatever shows up from the $PATH gets used.