Use standard autotools doxygen support.

The doxygen target directory is moved to the default location.

Generation of PDF doxygen documentation is enabled by default, can be
disabled with

$ ./configure --disable-doxygen-pdf

This uses ax_prog_doxygen.m4 which is part of the standard gnu
autoconf archive.  E.g.,

$ apt-get install autoconf-archive

A copy of /usr/share/aclocal/ax_prog_doxygen.m4 is put in m4/.

This is invoked in configure.ac.  It also contains a snippet for
inclusion in Makefile.am, which is extracted and included via
appropriate machinations in dox-make-dox-aminclude.am and invoked in
autogen.sh and Makefile.am..

One subtle tweak is a missing "touch timestamp file" line in an
automake stanza, which is overridden to avoid repeated re-running of
doxygen at "make" time, and also at "make install" time.
snip out dox-aminclude.am in a single pass using awk
include URL in configure.ac call to AC_INIT
support "pkg-config" by generating/installing .pc file
include otherwise-ignored stuff from git repo in "make dist"
Update git ignorance.

Git ignore doxygen debris.
Be more specific about our git ignorance.
Generally refresh the list.
Don't completely ignore libnet/m4/, as a file is cached there.
Ignore generated pkgconfig .pc file.