build: fix grep invocation in cc-options

build: fix grep invocation in cc-options

Currently the build produces lots of
        Usage: grep [OPTION]... PATTERN [FILE]...
        Try `grep --help' for more information.

This is due to the "grep -- $(2)" in cc-options. It seems that the
default of reading stdin is disabled when using "--". I don't know if
this is a bug in grep or how it is supposed to be but we can work
around it by explicitly passing in "-"

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>