filter: Fixes setting sample delay declaration for the PFB channelizer and in GRC.

Addresses issue #695.
filter: Fixes setting sample delay declaration for PFB synthesizer in GRC.

Addresses issue #695.
runtime: fix installation location of the gr_log_default.conf log configuration file.

Addresses issue #720.
cmake: Additional search path for QWT include files.

This is specific to QWT5. In 3.8, we will likely move to QWT6.

Addresses Issue #735.
audio: Fixes string format bug in ALSA error messages.

Addresses issue #736.
blocks: fixed relative rate in keep_m_in_n. It's m-in-n, not n-in-m.
Merge remote-tracking branch 'gnuradio-wg-grc/maint_grcwg' into maint