freebsd gettimeofday is not defined

[gperciva]

Nitpicky build failure on freebsd 10.3-RELEASE-p4. A normal make works just fine; doing clang -Weverything -Werror -Wno-a-bunch-of-specific-warnings produces this:

../libcperciva/util/monoclock.c:41:6: error: implicit declaration of function
      'gettimeofday' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if (gettimeofday(tv, NULL)) {

gettimeofday() is hidden inside #if __XSI_VISIBLE in /usr/include/sys/time.h. This isn't necessarily a huge problem since:

Applications should use the clock_gettime() function instead of the obsolescent gettimeofday() function.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html

and monoclock.c tries to use clock_gettime() first. However, in order to work around OSX failing:

All implementations shall support a clock_id of CLOCK_REALTIME as defined in <time.h>
http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_gettime.html

We defined -DPOSIXFAIL_CLOCK_REALTIME in Makefile.am in 00bf976.

There's obviously a few solutions we could use here:

• enable XSI stuff in the Tarsnap build.
• use more intelligent checking to see whether we need -DPOSIXFAIL_CLOCK_REALTIME or not.
• fancier code in monoclock.c. For example, scrypt selects clocktouse by going through CLOCK_VIRTUAL, CLOCK_MONOTONIC, CLOCK_REALTIME, and if all those fail, it bails. If that works on OSX, maybe we could do a similar thing in monoclock.c?
• stop testing with -Weverything -Werror. :)

[gperciva]

Fixed in d890938, when we enabled XSI stuff.