cgdb: update to 0.6.8

Changes:
- add missing build dependency to flex-native
- GDB is a runtime dependency, set it so
- use CACHED_CONFIGUREVARS to drop patch
- use ${BP}
- add missing DESCRIPTION
- no help2man build dependency (cgdb.1 man page is not generated)

Cc: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>