Arch Linux

I've attached a patch with some CMake improvements for the PKGBUILD.

These changes allow powerdevil to build with the Ninja CMake generator instead of only with the Unix Makefiles generator. While this won't be directly noticeable for direct users of powerdevil since the package is built by the maintainer, users building the package by themselves will benefit from these changes since they can use a different generator via the `CMAKE_GENERATOR` environment variable. Also, the powerdevil-light AUR package won't accept improvements unless they're adopted by the main powerdevil package as well. Since the powerdevil-light package currently fails to build when `CMAKE_GENERATOR` is set to anything other than "Unix Makefiles", I'm submitting these improvements so that the powerdevil-light package can adopt them as well which will allow it to build with any CMake generator.

More information on the latest CMake CLI features can be found here: https://cmake.org/cmake/help/v3.16/manual/cmake.1.html.