Various build fixes:

• Make sure man pages are always built correctly. They were only built if asciidoc happened to be installed and the output was XML instead of troff.
• Fix build on 13-CURRENT - the build picked up OpenMP implicitly and didn't link the library to libomp properly, which broke dependent ports (like graphics/ffmpeg).
• Fix build on releases if OpenMP was already installed.
• Enable building with OpenMP support by default on i386 and amd64.
• Bump revision

PR: 234285
Approved by: portmgr (build fix blanket)