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 <gitster@pobox.com>
Signed-off-by: Mike Ralphson <mike@abacus.co.uk>
Tested-by: Johannes Sixt <j6t@kdbg.org> (AIX 4.3.x)
Signed-off-by: Junio C Hamano <gitster@pobox.com>

preload-index.c

@@ -2,6 +2,14 @@
  * Copyright (C) 2008 Linus Torvalds
  */
 #include "cache.h"
+
+#ifdef NO_PTHREADS
+static void preload_index(struct index_state *index, const char **pathspec)
+{
+	; /* nothing */
+}
+#else
+
 #include <pthread.h>
 
 /*
@@ -81,6 +89,7 @@ static void preload_index(struct index_state *index, const char **pathspec)
 			die("unable to join threaded lstat");
 	}
 }
+#endif
 
 int read_index_preload(struct index_state *index, const char **pathspec)
 {