i18n: Do not force USE_GETTEXT_SCHEME=fallthrough on NO_GETTEXT

It should merely be the default used when the builder does not say
anything about USE_GETTEXT_SCHEME.

Even with NO_GETTEXT, USE_GETTEXT_SCHEME=gnu may be a way to avoid
possibly slower emulation in our shell scripts.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1515,7 +1515,7 @@ ifdef GETTEXT_POISON
 endif
 ifdef NO_GETTEXT
 	BASIC_CFLAGS += -DNO_GETTEXT
-	USE_GETTEXT_SCHEME = fallthrough
+	USE_GETTEXT_SCHEME ?= fallthrough
 endif
 ifdef NO_STRCASESTR
 	COMPAT_CFLAGS += -DNO_STRCASESTR