Remove ARDOUR::start_jack_server now that jack autostart works on windows
Remove windows specific quoting now that jack autostart works on windows

This allows jack2 to autostart using the .jackdrc we write
Add test for writing a jack config file
Fix for isnan using MinGW compiler
Ifdef some non-portable code for MinGW

jack_native_thread_t is HANDLE for MinGW builds of jack so using pthread API
cannot work.
Rename SearchPath class Searchpath

Windows headers define SearchPath which means we have to undefine it
where necessary. This is a pain and can be tricksy, so I feel renaming
the class slightly is the easiest solution.
Use Glib::usleep for portability
Only define certain debug flags when dist_target is auto

Fixes issue when cross compiling using MinGW
Use correct define when building ControlProtocol to export symbols