web: Get rid of doxygen.

Doxygen is an additional dependency, and it's total overkill for
a small project like osl. The self-contained gendoc.m4 script does
everything we need, and is much faster.

This patch converts osl.h.in to gendoc syntax and removes all traces
of doxygen. Since gendoc.m4 relies on features of gnu-m4 we run it with
-g to enable gnu extensions. This has been tested on FreeBSD and NetBSD
which ship different m4 implementations, both of which understand -g.