GH-13: man(1) on Mac OS X could not display URLs

The version of groff on Mac OS X is too old: it lacks the an-ext.tmac
macro package that defines styles for email addresses and general URLs.

    $ groff --version
    ...
    GNU grops (groff) version 1.19.2
    GNU troff (groff) version 1.19.2

This patch drops those URL macros in favor of simple angle brackets.