Log Message:
volk: update to 2.4.0

pkgsrc: since the volk release tarball only provides the required cpu_features
library as a git submodule that has to be manually cloned, we instead depend on
devel/cpu_features from pkgsrc and patch the cmake files to accept the static
library as an external dependency.

Documentation
  Update README to be more verbose and to improve usefulness.
Compilers
  AppleClang: Treat AppleClang as Clang.
  Paired with the cpu_features introduction, this enables us to use architecture
    specific kernels on a broader set of platforms.
CMake
  Enable to not install volk_modtool.
  Remove "find_package_handle_standard_args" warning.
cpu_features
  Use cpu_features v0.6.0 as a private submodule to detect available CPU
  features.
  Fix incorrect feature detection for newer AVX versions.
  Circumvent platform specific feature detection.
  Enable more architecture specific kernels on more platforms.
Kernels
  Disable slow and broken SSE4.1 kernel in volk_32fc_x2_dot_prod_32fc
  Adjust min/max for 32f_s32f_convert_8i kernel
  Use INT8_* instead of CHAR_*