app-doc/opengl-manpages: Bump to latest.

They have downloadable archives again, yay.  Still only gl2 manpages for
now.  There does not seem to be much point in keeping the old versions
around so let's punt them (I may not host the svn snapshots forever).

Convert the script to a makefile so that manpages can be built in
parallel which reduces installation time a lot on modern systems.