It's possible that this patch would cause breakage on other systems: 
there's a reason that errno is currently ignored, which is that it can't
be trusted on all systems (notably Linux: on one Linux system I've used
of three different evaluations, all of log singularity type (log(0),
log1p(-1), atanh(1), ...), one gave errno=EDOM, one gave errno=ERANGE,
and one didn't set errno at all).

So I'm reluctant to mess with math_1, especially this close to a release.

An alternative solution would be to check special cases for log directly
within the mathmodule.c code, only passing positive nonspecial arguments
to the system log function.  This would have a much lower risk of
causing breakage on other systems.  Note that Solaris with Sun's
compiler also has some problems with log:  depending on compiler
options, etc., log(-1.0) can give -inf instead of nan.  So having Python
deal with log special cases directly seems like a good thing to do there
as well.