PothosStandardFlags: set (Apple)Clang compile flags

[ncorgan]

This exposed some pretty legit warnings throughout.

[guruofquality]

Eventually this will be moved to lib where its a target_compile_options() that gets exported for clients who link the library

https://github.com/pothosware/SoapySDR/blob/master/lib/CMakeLists.txt#L47