Fixing CMake Compilation of meshfem2d/3d and specfem to account for single/double precision

[lsawade]

This issue was originally supposed to old the xgenerate_databases reader, but diverged due to other problems that need first to be fixed

• fortran_read_valye(type_real value, ....) read doubles and not type_reals
• CUSTOM_REAL not account for in meshfem2d/3d configuration.
• tests will not pass if simply creating
  • fortran_read_value(float value, ....)
  • fortran_read_value(double value, ....)