buildsystem: improve android and libraries detection

This patchset consider linux-android as a separate target than linux, removing the need to check for __ANDROID__ at preprocessor time to detect that we're building for android.

This enable fixing android buildsystem from the configure script in a saner way (for the other platforms) than autoconf tests, and enabling specific Android libraries from within the configure script.

This also adds a small build script for testing, for convenience for the reviewers, which can be stripped out from this MR.

This is a reworked partial re-send from !654.