Bug #1521947: possible bug in mystrtol.c with recent gcc.

Bug #1521947:  possible bug in mystrtol.c with recent gcc.

In general, C doesn't define anything about what happens when
an operation on a signed integral type overflows, and PyOS_strtol()
did several formally undefined things of that nature on signed
longs.  Some version of gcc apparently tries to exploit that now,
and PyOS_strtol() could fail to detect overflow then.

Tried to repair all that, although it seems at least as likely to me
that we'll get screwed by bad platform definitions for LONG_MIN
and/or LONG_MAX now.  For that reason, I don't recommend backporting
this.

Note that I have no box on which this makes a lick of difference --
can't really test it, except to note that it didn't break anything
on my boxes.

Silent change:  PyOS_strtol() used to return the hard-coded 0x7fffffff
in case of overflow.  Now it returns LONG_MAX.  They're the same only on
32-bit boxes (although C doesn't guarantee that either ...).