Merge Jens Rehsack's changes.

* thread safety and reentrant functions
* complete ports for HP-UX
* add support for AIX and Darwin/MacOS X
* add experimental support for tracing using log4cplus
* remove explicit Linux 2.4 support
* improve documentation
* add basic test capabilities

Thanks Jens!
Reapply support for FreeBSD 10.

This was introduced originally in commit
e655fcededb9b4040a8716b24ad8bc2155e31444 but got reverted in the mega
patch.
Whitespace cleanups.

Mostly using tabs instead of spaces to ident code, but also removing
some trailing whitespace from lines.
Add note to fix ChangeLog generation.
These aren't needed.
Must tell aclocal where our m4 files are.
Fix compiler error.

Fix empty va_list argument preparation for newer GNU compilers.
Adding disabled gettextize/autopoint.
Rename configure.in to configure.ac.
Eliminate some autoconf/automake warnings.
autoreconf does the job of all these tools now.

autogen.sh can probably be removed, but it serves as a good reminder
as to which command needs to be run.
Fix configure script.
Generate a ChangeLog file from the Git history.

This is a first attempt at producing something. I wanted to do it
without relying on an external tool since there doesn't seem to be a
readily packaged one (like cvs2cl). This output looks near enough to
the proper ChangeLog format to suffice.
Update configure.ac with changes from autoupdate.
Enforce building manual pages for maintainers.
Remove -Werror until the gettext stuff is fixed.

'warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and will soon be removed.'
Rename run_tests.pl.in to run_tests.t.in.
Renaming acx_*.m4 to ax_*.m4.
Rename ACX_* macros into AX_*, and remove gettext.
Reintroduce -Werror automake flag.