System directories when cross-compiling

Hi Marcin. Would you consider passing --enable-poison-system-directories to the cross compiler configure? This makes the '-Wpoison-system-directories' option available which warns you if the cross compiler picks up a library or header file from /usr instead of the cross-build environment.

I'm talking with someone who's looking at using the Linaro compiler and had a strange error due to picking up the host crtn.o. Having this warning would of tracked down the problem faster.

-- Michael