SDL or Allegro or something else instead?
I don't think so. Definitely not for v1.x
Might be a possibility for 2.0, but I have doubts that there will be runtimes other than DirectX. It's massive amounts of work.
Then again, some massive purple-skinned leprechaun might waddle into the source code and release a modified runtime in the current version of construct. It's open source, you never know.