section header for header section in configure.ac
git ignore libnet/compile (generated by autotools)
Use standard autotools doxygen support.

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

$ apt-get install autoconf-archive

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

The doxygen target directory is moved to the default location.

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

$ ./configure --disable-doxygen-pdf
Update git ignorance.

Git ignore doxygen debris.
Be more specific about our git ignorance.
Generally refresh the list.
include otherwise-ignored stuff from git repo in "make dist"
do not include generated man pages in distribution (because conditional!)
enable doxygen PDF documentation by default
avoid repeatedly rerunning doxygen