Remove libilbc-rfc3951 and msilbc submodules since the iLBC codec is now included in the mswebrtc submodule.

Move antlr3, cunit, gsm, matroska, opencore-amr, opus, polarssl, speex, srtp, vo-amrwbenc and xml2 submodules to externals.

- Derive package version number from git version
- Include xml documentation file in package if present
- Handle build in Release mode