Support custom build options in config.mak

With this patch, it is possible to store configuration options like
NO_CURL=YesPlease or NO_OPENSSL=YesPlease into a file named
config.mak, which will be included in the Makefile.

[jc: redone with suggestion from Daniel Barkalow to just use -include]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -210,6 +210,8 @@ ifneq (,$(findstring arm,$(uname_M)))
 	ARM_SHA1 = YesPlease
 endif
 
+-include config.mak
+
 ifndef NO_CURL
 	ifdef CURLDIR
 		# This is still problematic -- gcc does not want -R.