intprops: check two's complement assumption Suggested by Eric Blake in: http://lists.gnu.org/archive/html/bug-gnulib/2016-04/msg00016.html * lib/intprops.h: Include <verify.h>. Verify that signed char, short, int, long, and (if available) long long are two's complement. * modules/intprops (Depends-on): Add 'verify'.