support comedi whenever found

Remove the LINUX dependency from the gr-comedi CMake rules,
as, for example, comedi also has support on Debian kFreeBSD
which is not a LINUX platform.
find USB on all platforms

Keep the 'usb-1.0' as the preferred match, but also match simply
'usb' as provided on some platforms. Allows kFreeBSD system support.
Merge branch 'maint'
Merge branch 'master' into next
Updated to revision 3.7.4.1 for release.
Merge branch 'maint'

Used merge strategy -s ours to skip release commit.
Updated revision to 3.7.5 for release.
Updated revision post-release.
Merge branch 'master' into next

Used merge strategy -s ours to skip release commits.
volk: adding tanh kernel with support for sse and avx.

There are two generic kernels, one using tanh and another using a series approximation. The SIMD code uses the series approximation.
Merge branch 'master' into next