Makefile: introduce NO_PTHREADS

Makefile: introduce NO_PTHREADS

This introduces make variable NO_PTHREADS for platforms that lack the
support for pthreads library or people who do not want to use it for
whatever reason.  When defined, it makes the multi-threaded index
preloading into a no-op, and also disables threaded delta searching by
pack-objects.

Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Mike Ralphson <redacted>
Tested-by: Johannes Sixt <redacted> (AIX 4.3.x)
Signed-off-by: Junio C Hamano <redacted>