Simplify creation of Fortran function wrappers

The old version invoked a Fortran compiler to figure out the bindings
necessary to link to a Fortran function. This creates a dependency on
having a Fortran compiler, even though the project may not have any
Fortran source it needs to compile!

Also, the Fortran compiler that is installed on the system may not be
the same as was used to compile the library anyway, so we are not even
sure that this is correct!

Furthermore, FindLAPACK operates on the assumption that names in that
library is suffixed with a single underscore, so if that is not correct,
we won't find any of the functions in that library anyway!

Thus, this patch enables us to assume that appending an underscore is
the right thing to do without using the compiler. This option is off by
default, but can be activated with USE_UNDERSCORING=ON (named after the
GFortran option).