Standalone mode and EV_USE_REALTIME

[ghudson at MIT.EDU]

As of 4.04 and trunk, if ev.c is built in standalone mode with no
prior defines, EV_USE_REALTIME is defined to 1 unless
EV_USE_CLOCK_SYSCALL is guessed.  This results in a dependency on
clock_gettime().  Is that intended or a bug?  Normally the code tries
to be conservative about OS functionality in standalone mode.

(On Solaris 10, this results in a dependency on librt unless
EV_USE_REALTIME is overridden.)