Improve build system auto-selection process

Improve build system auto-selection process

This patch alters semantics of check_auto_buildable() a bit. Now it can
also indicate if the source has already been partitially built with the
build system and if so, such build system may be auto-selected over a less
specific its parent (in the inheritance tree) even if the latter is earlier
in the @BUILDSYSTEMS array.

However, this still leaves a requirement that a derivative build system
must not do anything that may break packages of the parent build system.
Otherwise, introduction of a new derivative build system might break
packages which already had that build system implemented via overrides...

Signed-off-by: Modestas Vainius <modestas@vainius.eu>