Note than only "setting" CC=gcc CXX=g++ will not work:
CC=gcc
CXX=g++
sh ../js/src/configure.in ...
will not pass CC=gcc and CXX=g++ to sh. It's needed to export CC and
CXX:
CC=gcc
CXX=g++
export CC CXX
sh ../js/src/configure.in ...
Or, explicitly pass them:
CC=gcc CXX=g++ h ../js/src/configure.in ...
firefox-91.9.1,
js-91.9.1,
thunderbird-91.9.1.
The smaller SBU times on firefox and js are because my SBU with
gcc-12 takes longer, not because the builds are quicker.