Log Message:
Disable (on NetBSD) ill-conceived GNUish mktime() code that generates
a compile-time assertion. There is no need for this code on NetBSD, or
probably anywhere else, as mktime() is a standard part of libc.

The broken code was apparently reported upstream last year, with
predictable results: http://sourceware.org/bugzilla/show_bug.cgi?id=12401

While here also fix the mktime configure test, which exercises
undefined behavior. With gcc 4.5 this causes the build to hang in an
infinite loop for sixty seconds until an alarm() goes off.

Fixes the build on 32-bit NetBSD.