Apple M1 build recipe for 0.2.7

[mlooney]

Hello,

I recently built CubicSDR from source, and had to make a few workarounds to get things to build (based on macOS build wiki):

1. if installing liquid-dsp from source, with the default --prefix (/usr/local), CMakefiles/CubicSDR.dir/flags.make needs -I/usr/local/include/liquid added to the CXX_INCLUDES
2. having built SoapyRTLSDR from source, it also installed to /usr/local, requiring -DBUNDLE_SOAPY_MODS=1 -DSOAPY_SDR_ROOT=/usr/local to be added to the cmake args
3. the new code signing regime on osx will cause the application to immediately exit due to not being signed, with the exception type Exception Type: EXC_BAD_ACCESS (SIGKILL (Code Signature Invalid)). I worked around this by executing codesign --force --deep --sign - x64/CubicSDR.app to use ad-hoc code signing.

This will resolve the "no soapy modules found" dialog, which I leave here so that others might find this :)