Darwin: Ignore missing /sw/lib

When on Darwin platforms don't include Fink or DarwinPorts
into the link path unless the related library directory
is actually present.  The linker on MacOS 10.4 complains
if it is given a directory which does not exist.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -223,11 +223,15 @@ ifeq ($(uname_S),Darwin)
 	NEEDS_SSL_WITH_CRYPTO = YesPlease
 	NEEDS_LIBICONV = YesPlease
 	## fink
-	ALL_CFLAGS += -I/sw/include
-	ALL_LDFLAGS += -L/sw/lib
+	ifeq ($(shell test -d /sw/lib && echo y),y)
+		ALL_CFLAGS += -I/sw/include
+		ALL_LDFLAGS += -L/sw/lib
+	endif
 	## darwinports
-	ALL_CFLAGS += -I/opt/local/include
-	ALL_LDFLAGS += -L/opt/local/lib
+	ifeq ($(shell test -d /opt/local/lib && echo y),y)
+		ALL_CFLAGS += -I/opt/local/include
+		ALL_LDFLAGS += -L/opt/local/lib
+	endif
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease