On Solaris choose the OLD_ICONV iconv() declaration based on the UNIX spec

OLD_ICONV is only necessary on Solaris until UNIX03.  This is indicated
by the private macro _XPG6 which is set in /usr/include/sys/feature_tests.h.

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

Makefile

@@ -705,7 +705,7 @@ ifeq ($(uname_S),SunOS)
 	NO_STRCASESTR = YesPlease
 	NO_MEMMEM = YesPlease
 	NO_MKDTEMP = YesPlease
-	OLD_ICONV = UnfortunatelyYes
+	NO_MKSTEMPS = YesPlease
 	ifeq ($(uname_R),5.8)
 		NO_UNSETENV = YesPlease
 		NO_SETENV = YesPlease

utf8.c

@@ -354,7 +354,7 @@ int is_encoding_utf8(const char *name)
  * with iconv.  If the conversion fails, returns NULL.
  */
 #ifndef NO_ICONV
-#ifdef OLD_ICONV
+#if defined(OLD_ICONV) || (defined(__sun__) && !defined(_XPG6))
 	typedef const char * iconv_ibp;
 #else
 	typedef char * iconv_ibp;