add PBD::downcase() functions to libpbd
required header for new search_paths.h
use PBD::downcase() rather than non-portable MS-based strlwr()
use buf_index_in() to identify VST MIDI buffer

Probably unnecessary as long we only have 1 MIDI port route, but this at least keeps symmetry with the output side
fix up waf build in wake of search path changes earlier today