* Add support for building with clang.
* Enable 4k@60 fps by default (doesn't hurt on kms even if using lesser
resolutions).
Note that not all of the Arch ARM builders have enough RAM to support LTO.
Recommend manually setting -DUSE_LTO=OFF to -DUSE_LTO=$(nproc) in build() in
order to benefit from clang's -flto=thin switch. Do this even if building with
gcc since LTO there has been enabled for sometime.