Bug 489643 - set sane default options in UNPACK_TAR; r=benjamin

toolkit/mozapps/installer/packager.mk

@@ -87,7 +87,7 @@ MAKE_PACKAGE	= $(error What is a $(MOZ_PKG_FORMAT) package format?);
 
 CREATE_FINAL_TAR = $(TAR) -c --owner=0 --group=0 --numeric-owner \
   --mode="go-w" -f
-UNPACK_TAR       = tar -x
+UNPACK_TAR       = tar -xf-
 
 ifeq ($(MOZ_PKG_FORMAT),TAR)
 PKG_SUFFIX	= .tar