fix requirement of existing ChangeLog for autoconf

Add a rule to generate a ChangeLog (make ChangeLog) to avoid the file
must exist before auto(re)conf is invoked
prefer AS_IF over handcrafted if/then/fi
introduce STATGRAB(LIBS|CPPFLAGS) for additional

while statgrab itself gets more complex, requirements becoming tricky, too.
introduce common place to merge build requirement check results and it's
first customer is failing isnan symbol on AIX when lining statgrab.
fix ChangeLog is tried to rebuild for non-devs

Only maintainer shall rebuild ChangeLog - all the rest should take the
deployed one or dig deeper.
Use STATGRABCPPFLAGS.

This is exported by the configure script so we should use it here.
Even if it has nothing in it now it'll avoid confusion later.
statgrab.h should not be bundled in dist

It's generated from statgrab.h.in.
Update for 0.91-rc2.
Update for 0.91 release.
deal with broken procfs

On several embedded platforms the procfs implementation might differ.
Deal with that correctly (fix fd resource leak).
fix parallel build of manpages and symlinks

for GNU make and some BSD makes. Typically people won't encounter it,
because they use the distributed package which already contains manpages.
Other makes can be supported once concrete issues pop up.
fetch thread name from comm, if available

Since sg_get_process_stats eliminates surrounding brackets either when
comm (thread name of active thread of a process) is taken from stat
file, read it pure from comm.
Because of some firmware drivers create names containing spaces, this is
safer than parsing unknown number elements in front of stat content.
Add an internal script for generating HTML docs.

This script isn't intended for distribution, but I wanted to put it
somewhere it wouldn't get lost. It can be run after the manual pages
have been built to generate HTML versions suitable for putting on the
website.