Don't FTBFS if config.mk doesn't exist

Don't FTBFS if config.mk doesn't exist

Origin: vendor
Bug-Debian: http://bugs.debian.org/720781
Forwarded: no

The config.mk file isn't guaranteed to exist (it gets removed by the Makefile's
"distclean" target). Including it with a straight "include" causes cmus to
FTBFS on the initial clean, since it doesn't exist and make aborts because it
can't include it. Instead, include it using "-include", which ignores it if it
doesn't exist.
Forwarded: no
Gbp-Pq: Name 01_config.mk.diff