This adds support for building foreign libraries with a given ABI version on Linux. This is enables foreign libraries to specify ABI compatibility information. This is important since ABI compatibility differs from package release versions.

Two new fields are added: lib-version-info and lib-version-linux. The former accept versions Libtool-style, the latter sets SONAME versions directly. In both cases, appropriate symlinks are installed.

Libtool accepts ABI version data via the -version-info flag, which takes current[:revision[:age]] data. This is then parsed into a major.minor.build version number. We copy this approach so that library versioning may be generalised to other operating systems than Linux.