[PATCH] Makefile: introduce NO_PTHREADS

[Junio C Hamano <gitster@pobox.com>]

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>
---

 * I notice a handful platforms do not define THREADED_DELTA_SEARCH, and
   on them Linus's preload-index.c is the first source file that includes
   <pthreads.h>, which may result in breakages.

 Makefile        |   10 +++++++++-
 config.mak.in   |    1 +
 configure.ac    |    5 +++++
 preload-index.c |    9 +++++++++
 4 files changed, 24 insertions(+), 1 deletions(-)

diff --git c/Makefile w/Makefile
index acac0ae..ffc9531 100644
--- c/Makefile
+++ w/Makefile
@@ -90,6 +90,8 @@ all::
 #
 # Define NO_MMAP if you want to avoid mmap.
 #
+# Define NO_PTHREADS if you do not have or do not want to use Pthreads.
+#
 # Define NO_PREAD if you have a problem with pread() system call (e.g.
 # cygwin.dll before v1.5.22).
 #
@@ -1018,9 +1020,15 @@ ifdef INTERNAL_QSORT
 	COMPAT_OBJS += compat/qsort.o
 endif
 
+ifdef NO_PTHREADS
+	THREADED_DELTA_SEARCH =
+	BASIC_CFLAGS += -DNO_PTHREADS
+else
+	EXTLIBS += $(PTHREAD_LIBS)
+endif
+
 ifdef THREADED_DELTA_SEARCH
 	BASIC_CFLAGS += -DTHREADED_DELTA_SEARCH
-	EXTLIBS += $(PTHREAD_LIBS)
 	LIB_OBJS += thread-utils.o
 endif
 ifdef DIR_HAS_BSD_GROUP_SEMANTICS
diff --git c/config.mak.in w/config.mak.in
index ea7705c..14dfb21 100644
--- c/config.mak.in
+++ w/config.mak.in
@@ -51,4 +51,5 @@ OLD_ICONV=@OLD_ICONV@
 NO_DEFLATE_BOUND=@NO_DEFLATE_BOUND@
 FREAD_READS_DIRECTORIES=@FREAD_READS_DIRECTORIES@
 SNPRINTF_RETURNS_BOGUS=@SNPRINTF_RETURNS_BOGUS@
+NO_PTHREADS=@NO_PTHREADS@
 PTHREAD_LIBS=@PTHREAD_LIBS@
diff --git c/configure.ac w/configure.ac
index 4256742..8821b50 100644
--- c/configure.ac
+++ w/configure.ac
@@ -490,6 +490,8 @@ AC_SUBST(NO_MKDTEMP)
 # Define NO_SYMLINK_HEAD if you never want .git/HEAD to be a symbolic link.
 # Enable it on Windows.  By default, symrefs are still used.
 #
+# Define NO_PTHREADS if we do not have pthreads
+#
 # Define PTHREAD_LIBS to the linker flag used for Pthread support.
 AC_LANG_CONFTEST([AC_LANG_PROGRAM(
   [[#include <pthread.h>]],
@@ -502,9 +504,12 @@ else
  ${CC} -lpthread conftest.c -o conftest.o > /dev/null 2>&1
  if test $? -eq 0;then
   PTHREAD_LIBS="-lpthread"
+ else
+  NO_PTHREADS=UnfortunatelyYes
  fi
 fi
 AC_SUBST(PTHREAD_LIBS)
+AC_SUBST(NO_PTHREADS)
 
 ## Site configuration (override autodetection)
 ## --with-PACKAGE[=ARG] and --without-PACKAGE
diff --git c/preload-index.c w/preload-index.c
index 6253578..3ae83dc 100644
--- c/preload-index.c
+++ w/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)
 {