Simplify `nginx -V` output

This is filed into the category known as "nitpicking, the final frontier", but I thought it might be useful to slightly alter the output of `nginx -V`, removing useless workdir folders.

Turns
`--add-module=/var/tmp/portage/www-servers/nginx-1.7.3/work/nginx_mogilefs_module-1.0.4`

...to
`--add-module=external_module/nginx_mogilefs_module-1.0.4`

I don't particularly see any side effects here. No one would be able to snag that output and compile from scratch anyway. Also, it might more sense to people debugging outside of the gentoo community.