Fatal error when using cmake < 2.8.10 on ARM NEON

Address issue #733: This makes the known issue of building VOLK for ARM NEON with
CMake version < 2.8.10 a fatal error at cmake time, and gives a message to that effect.
This is considered preferable to a linking error towards the end of the build.
blocks: convert asserts to throwing runtime errors in keep_m_in_n ctor.
blocks: fixed up keem_m_in_n to handle relative rate and correct number of items in calls to work.

Original patch from Zhe Feng on mailing list.
Merge branch 'maint'
Merge remote-tracking branch 'dgeiger/cmake_neon'
build_guide: Make explicit we only support Qt4
Merge branch 'maint'
uhd: Amends 2fd13c0c, LO lock checks need to be removed from source as well as sink
blocks: throttle resets the timer state when the topblock starts each block, including during a lock/unlock restart.

Addresses issue #649.
docs: Adds a brief explanation about adding and getting items tags inside/outside calls to work.

Addresses issue #699.
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: switching last calls from boost::thread to gr::thread.

There are a few places where the threads are specifically used by boost::thread calls in the namespace. If we ever move away from using boost's threads, we might have some more specific cleanup to do, but it's not obvious how to easily abstract this further.

Addresses Issue # 706.
Merge remote-tracking branch 'mbant/uhd/even_less_checks'
grc: making qt-gui the default (Feature #634)
grc: show fg return code if not 0 (Bug #726)
grc: switch timestamp in grc files from modified to created