always build shared libraries

Always build shared libraries and always call the DMTX library `libdmtx.`_ext_.

There may be travis-ci failures in [csvimp](https://github.com/xtuple/csvimp) in [qt-client](https://github.com/xtuple/qt-client) after this is merged but before the corresponding changes are merged in those repos.

@dpocock - could you review this PR to make sure your needs are met?