linuxptp: don't check host headers for target features

linuxptp: don't check host headers for target features

The incdefs.sh script runs ${CROSS_COMPILE}cpp to get a list of default
headers directories. Make sure CROSS_COMPILE is set correctly to avoid check
of host headers.

Also, drop the CC command line override. Upstream makefile sets CC to
$(CROSS_COMPILE)gcc.

Should fix:
http://autobuild.buildroot.net/results/394/394414df25b0b7261b27ca5d4901fde0f639e73c/
http://autobuild.buildroot.net/results/6ad/6adbc8c77c2ca70246b31600ab66b37c56c1e89e/
http://autobuild.buildroot.net/results/70d/70dfd05acc9d951169267f2820ecedb37f0711fe/

Cc: Petr Kulhavy <brain@jikos.cz>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>