Add a no-op setitimer() wrapper

Add a no-op setitimer() wrapper

The current code uses setitimer() only for reducing perceived
latency.  On platforms that lack setitimer() (e.g. HP NonStop),
allow builders to say "make NO_SETITIMER=YesPlease" to use a no-op
substitute, as doing so would not affect correctness.

HP NonStop does provide struct itimerval, but other platforms may
not, so this is taken care of in this commit too, by setting
NO_STRUCT_ITIMERVAL.

Signed-off-by: Joachim Schmitz <jojo@schmitz-digital.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>