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'