Makefile: avoid error message from 'uname -o'

Makefile: avoid error message from 'uname -o'

The platform specific tweaking part was using 'uname -o' which
is not always available.  Squelch error message from it.

It was suggested to chain the if..else, but I chose not to, because
maintaining the nested if..else if..else..endif endif to match is a
pain.  If we had "elif", things would have been different, though.
While we are at it, try not to invoke 'uname -s' for each platform
candidate.

Signed-off-by: Junio C Hamano <junkio@cox.net>