gr-uhd: add option to uhd_rx_cfile to write metadata file.
volk: make volk's get_machine thread safe.

Sets a local struct instead of the global while searching. When found, then sets the machine.

This assumes that the pointer assignment is atomic, which it probably is on almost any platform.
Merge branch 'maint'
volk: follow-up to commit cb4142d45 for added volk_get_machine function on master.
Merge remote-tracking branch 'stiabhan/uhd_rx_cfile'
cmake: fixed ICE 3.4 version check for GCC < 4.7.
volk: remove unused spu_lib directory from VOLK.

Addresses Issues #638.