Makefile: define __sun__ on SunOS

The SUNWspro compiler does not define __sun__ (like GCC does).  A check of
this macro was recently added to detect compilation on SunOS and to modify
the handling of the NO_ICONV and _XOPEN_SOURCE feature macros.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -723,7 +723,7 @@ ifeq ($(uname_S),SunOS)
 	endif
 	INSTALL = /usr/ucb/install
 	TAR = gtar
-	BASIC_CFLAGS += -D__EXTENSIONS__
+	BASIC_CFLAGS += -D__EXTENSIONS__ -D__sun__
 endif
 ifeq ($(uname_O),Cygwin)
 	NO_D_TYPE_IN_DIRENT = YesPlease