preserving compiler flags for fortran

The current monkey-patch for compiling FORTRAN does not pass the compiler options determined by setuptools nor the `CFLAGS` environment variable. Disadvantageous:
* no easy customization of the build process via `CFLAGS`. E.g. hardening options set by debhelper during debian packaging are not propagated to gfortran (@megies)
* currently we use no optimization, this is now enabled by default

The following patch simplifies the FORTRAN monkey-patch and preserves the above mentioned flags.