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.