setting gcc compiler in pybombs

Hail,
What is the recommended method to set the gcc compiler used by
PyBOMBS in building with gnuradio.lwr (gitbranch: maint-3.9)?
I am trying to trace back problems but first should use 8.2.1 (installed
as /usr/bin/gcc-8) as opposed to the system gcc which is 7.5.0. I
assume that CMAKE_CXX_COMPILER_VERSION must be defined somewhere.

Rick