Allow shortcutting builds

[ktf]

This shows how the alibuild-variant-support package could be used to e.g. speed up builds of O2Physics tutorials by only building the Tutorials/install target, rather than the whole package.

[ktf]

@aalkin @TimoWilken this is what I think I was mentioning last week / in the past.

Right now it would only work with:

ALIBUILD_BUILD_VARIANT=tutorial aliBuild build O2Physics --defaults o2 --debug

but it would be cool to have support in aliBuild so that one could do:

aliBuild build O2Physics#tutorials --defaults o2 --debug

with the "#tutorial" being stripped to determine the package name, yet inserted in the develHash, so that if one drops it, it rebuilds the development package without need to cleanup.

[awegrzyn]

Are there any docs on how to use ALIBUILD_BUILD_VARIANT? It seems like interesting feature.

[ktf]

not yet, we are still developing the idea. we could brainstorm things a bit together though, since indeed you / FLP was one of the customers I had in mind for such a feature.