Set default FORTRAN standard to F77

With an up-to-date gfortran version compilation fails because some constructs are not allowed anymore in the more modern Fortran standards, which are default in gfortran now, e.g. things like

Warning: Fortran 2018 deleted feature: DO termination statement which is not END DO or CONTINUE with label 3 at (1)

or

cd /home/maximilian/git/crmc/build/src/sibyll && /usr/bin/gfortran -DSibyll_EXPORTS -DWITH_HEPMC -I/usr/include -I/home/maximilian/git/crmc/src -I/home/maximilian/git/crmc/src/sibyll -Wunused -O -Wuninitialized -fPIC -D __SIBYLL__ -c /home/maximilian/git/crmc/src/sibyll/sibyll_epos.f -o CMakeFiles/Sibyll.dir/sibyll_epos.f.o
/home/maximilian/git/crmc/src/sibyll/sibyll_epos.f:549:22:

  315 |               if(drangen(dummy).lt.0.45d0) then
      |                         2
......
  549 |       S_RNDM= drangen(dble(idum))
      |                      1
Error: Type mismatch between actual argument at (1) and actual argument at (2) (REAL(8)/REAL(4)).

To fix these issues I added the -std=legacy compiler flag for gfortran.