build: add LAZY_LOAD_LIBS option and always link with threads
build: add LAZY_LOAD_LIBS option and always link with threads

Users can now disable the lazy loading of libraries, and use classic
linkage instead.

Previously, cubeb would link with threads only if using some backends.
This caused linking issues when no backends were active, because cubeb
wasn't linked against pthread or similar, while the library is needed by
src/cubeb_log.cpp, as it uses std::thread.

 #cleanup

kinetiknz/cubeb,kinetiknz/cubeb,kinetiknz/cubeb