autoconf.mk.dep: use target cflags, not host

The current autoconf.mk.dep rule uses the host cflags when executing the
target compiler (which includes target header files).  We don't want to
mix the target compiler and host compiler flags, so change it to CFLAGS.
Otherwise we get things like -pedantic which the U-Boot source code does
not build with.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>