This checks for some system macros to be set, but also includes the
doxygen build to generate documentation.
This reuses the RADIOLIB_EXCLUDE_STM32WLX that the code already checks
for to prevent having to duplicate this macro check in four places.
To detect running inside doxygen, this configures doxygen to predefine
a DOXYGEN macro (it seems no such macros are defined by default by
doxygen).