All systems with sys/timex.h seem to have a subset of the STA_ symbols

We therefore don't need to test for all of them.  Solaris, for example is
missing, STA_NANO, STA_CLK, and STA_MODE.

These 3 symbols were added to glibc in March 2008 per
https://sourceware.org/ml/libc-alpha/2008-03/msg00076.html