Int64 build eclass

It fails though without the export of FCFLAGS:

````
checking whether we are using the GNU Fortran compiler... yes
checking whether x86_64-pc-linux-gnu-gfortran accepts -g... yes
checking how to get verbose linking output from x86_64-pc-linux-gnu-gfortran... -v
checking for Fortran libraries of x86_64-pc-linux-gnu-gfortran...  -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1 -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/../../../../x86_64-pc-linux-gnu/lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/../../.. -lgfortran -lm -lquadmath
checking for dummy main to link with Fortran libraries... none
checking for Fortran name-mangling scheme... unknown
configure: error: Could not figure out Fortran name mangling scheme.
````