Makefile: Warn if user tries to compile for libpayload w/o crossgcc.

While flashrom is not as picky on compilers as coreboot, there is still a
high probablilty of breakage when one combines libpayload and distribution
compilers. Print a warning if we detect that to give the daring user a hint
how to resolve the explosions potentially following below it.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1740 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1