Makefile: Add static check for misuse of LITTLE_ENDIAN and BIG_ENDIAN.

I've pointed out two misuses of these macros in review in the last week,
by different authors.  It's time to make it difficult to screw this up.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>