Add BUILD_MESSAGE() macro

This commit introduces the BUILD_MESSAGE() macro. It uses _Pragma("message"),
with compilers that support that, to output a warning-like compile-time message
without blocking the compilation.

Used by next commit.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
lib/flow: Use BUILD_MESSAGE() to warn if BUILD_SEQ is not updated

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>