mk: fix config build on freebsd
mk: fix config build on freebsd

The '-fdirectives-only' flag doesn't exist in cpp of FreeBSD.
To remove expanding macros when parsing the config files won't
add any issue.

Suggested-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>
Signed-off-by: Tetsuya Mukawa <b347a9f5840345582b30d055e1e99ac6ac7bce3c@igel.co.jp>
Acked-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>

venkynv/dpdk-mirror,mixja/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,msune/dpdk,msune/dpdk,mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork