An optional dependency, Vc, doesn't currently provide any optimisations
on aarch64 and causes a build failure in krita. Exclude it from the
build inputs on arm and aarch64.
The version of LCMS bundled with opencolorio is too old to build on
aarch64, simply because its config files date from before aarch64
was announced. However, it can use the system lcms2 if it is found.
Also de-vendor tinyxml.
In addition, the version had been bumped to 1.1.0, but 1.0.9 was still
hard-coded in the fetch uri. Some changes were necessary for 1.1.0 to build.
As the sources are fetched from github, use fetchFromGitHub instead of
fetchurl.
On aarch64, linking against the vendored ffmpeg fails. Including ffmpeg
as a dependency and passing -system-ffmpeg to qmake fixes this.
Slightly odd conditional in qmakeFlags to avoid altering the list on
non-arm platforms, so that the change doesn't trigger an unneccessary
rebuild.
Tom Hall