Support bulding with external siod and rxp

Both components are used by other software and may already be present on one's system. It'd be nice, if it were possible to build speech_tools against the already-installed versions of [siod-3.6.x](http://people.delphiforums.com/gjc/siod.html) and [rxp-1.5.2](https://www.cogsci.ed.ac.uk/~richard/rxp.html) instead of the bundled code.