blocks: correcting use of mutexes in file_meta_sink.

the gr::block class has a d_setlock mutex that's appropriate for this use instead of creating another for the child class.
qtgui: fixed a bug in the constellation plots that didn't allow us to reduce the number of samples.
qtgui: replaced use of local mutex with d_setlock from gr::block.

Partial response to issue #706.
trellis: minor fixups to trellis examples so prefix doesn't need to end with '/'.
docs: added some cmake cli flags to better configure the build.
Merge branch 'maint'