external: import cpu_features library

Taken from https://github.com/google/cpu_features at revision
26133d3b620c2c27f31d571efd27371100f891e9

external/cpu_features/cmake/README.md

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+# CMake build instructions
+
+## Recommended usage : Incorporating cpu_features into a CMake project
+
+  For API / ABI compatibility reasons, it is recommended to build and use
+  cpu_features in a subdirectory of your project or as an embedded dependency.
+
+  This is similar to the recommended usage of the googletest framework
+  ( https://github.com/google/googletest/blob/master/googletest/README.md )
+
+  Build and use step-by-step
+
+
+  1- Download cpu_features and copy it in a sub-directory in your project.
+      or add cpu_features as a git-submodule in your project
+
+  2- You can then use the cmake command `add_subdirectory()` to include
+     cpu_features directly and use the `cpu_features` target in your project.
+
+  3- Add the `cpu_features` target to the `target_link_libraries()` section of
+     your executable or of your library.
+
+## Enabling tests
+
+  CMake default options for cpu_features is Release built type with tests
+  disabled. To enable testing set cmake `BUILD_TESTING` variable to `ON`,
+  [.travis.yml](../.travis.yml) and [appveyor.yml](../appveyor.yml) have up to
+  date examples.