Yes not sure how those temporary cmake files work.
Actually, when did you last run the cmake
command (cmake /extrabela/supercollider -DCMAKE_C_COMPILER=/usr/lib/ccache/gcc-4.8 -DCMAKE_CXX_COMPILER=/usr/lib/ccache/g++-4.8 -DNOVA_SIMD=ON -DSSE=OFF -DSSE2=OFF -DINSTALL_HELP=OFF -DSC_QT=OFF -DSC_IDE=OFF -DSC_EL=OFF -DSC_ED=OFF -DSC_VIM=OFF -DSUPERNOVA=OFF -DNO_AVAHI=ON -DNATIVE=ON -DENABLE_TESTSUITE=OFF -DAUDIOAPI=bela
or similar) ?
If that was when you had an older version of the Bela code (i.e.: before October 30), then maybe you should just re-run it and it the build may just work.
If that does not, then you may find out that the easiest ( and almost* correct) way of doing this is to edit the supercollider/cmake_modules/FindBela.cmake
file and append -lmathneon
to line 68.
* I say "almost" because I would expect that the former solution should just work and I am not sure why it does not.