Remove deprecated AM_MAINTAINER_MODE from configure.ac.

See
www.gnu.org/software/automake/manual/html_node/maintainer_002dmode.html#maintainer_002dmode
for the rationale.  Summary:

  ... François Pinard pointed out several arguments against this
  ... macro ... Jim Meyering, the inventor of the AM_MAINTAINER_MODE
  macro was swayed by François’s arguments, and got rid of
  AM_MAINTAINER_MODE in all of his packages.
Accept result of autoupdate.
Remove unneeded file include/stamp-h.in.
Add section header for header section to configure.ac.
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 via 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.
Ignore libnet/compile, as it is generated by autotools.
Don't completely ignore libnet/m4/, as a file is cached there.
Ignore generated pkgconfig .pc file.