On some systems (notably HPUX), there is already a
/usr/include/sys/_time.h, which our sys/_time.h shadows. Found and
diagnosed by Kathryn Hogg.
This is a quick fix for 1.4 only; for 2.0, I want to eliminate
compat/sys/_time.h entirely, and have util-internal subsume it.
svn:r1493
