lang/gawk: update the port to the latest version 5.2.2

GNU make is not required for the build, but the test suite assumes
this particular implementation in several places. While they could
be patched, some of these changes won't be easily upstreamable I'm
afraid, so let's just USES+=gmake for the time being.

Drop the `post-patch' target added in commit 0c0c5d528f4cd (n243249)
to prevent bogus makeinfo(1) banner propagation as this problem no
longer exhibits itself.

Reported by: portscout