sudo: set CFLAGS for build mksigname and mksiglist

sudo: set CFLAGS for build mksigname and mksiglist

mksigname and mksiglist are compiled by BUILD_CC for build host. When
there are some options in CFLAGS that BUILD_CC doesn't support,
compilation fails.

Build for arm on a x86 host, if option "-mapcs-frame" is provided, error
occurs with:

| cc1: error: unrecognized command line option "-mapcs-frame"

Pass BUILD_CFLAGS to CFLAGS to fix that kind of failure.

(From OE-Core rev: 6022e81570b80232f272a1aa474e8ced3a089382)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>