Missing stdbool.h for 'bool' in features .h, at least one user is arduinoWebSockets
Adds minimal headers sanity-check script to verify that C builds work as expected
Also noticed and removed default argument from crc32() in internal .h that may be used in .c
(not sure how extern C & default worked simultaniously, but at least in our .cpp Gcc somehow figured out it is a no overload solution)
