Make defaults sensible for windows, fix program finding.

[MHebes]

Thanks for the project! Just some things I came across when compiling on windows.

Uses find_program instead of set to find avr tools so that .exe suffix is added. Small behavior change: Does not FORCE the cache variable change. Could get around this with a find_program(NO_CACHE) followed by set(CACHE FORCE) if you'd like, it just looks a little clunkier.

Checks CMAKE_HOST_WIN32 to give a different default for ARDUINO_PATH.

Mentions issue about Visual Studio generators not respecting CMAKE_CXX_COMPILER.