tweak backends wscript for cross-compiling with mingw
copy over current Waves version of their backend, along with minor changes in libs/ardour and libs/backend/jack to fit with API changes
Re-introduce our pthread macros (after fixing an earlier typo)

these are so we can differentiate between 'libpthread' and 'libpthread-win32' (whose implementation is subtlely - though annoyingly! - different)
Implement the new pthread macros (for the Waves audio backend)