With the recent addition of a check for directories listed in Makefile
local custom directories are always reported as unlisted. This patch
adds support for a “Makefile.local” file, which can adjust settings in
Makefile. Example: “DIRCHECK_EXCLUDE += xyz .mydata doc/manhtml”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>