Issue description

Building Fortran projects currently leads to a massive amount of warning messages

f951: Warning: command line option '-Wformat=1' is valid for C/C++/ObjC/ObjC++ but not for Fortran
f951: Warning: command line option '-Wformat-security' is valid for C/C++/ObjC/ObjC++ but not for Fortran

See e.g. liblapack's log output
This is caused by the format hardening adding those flags to NIX_CFLAGS_COMPILE, which in turn is used also for Fortran builds.

This also leads to the breakage of a build system of one package I try to compile, as it expects gfortran not to produce any warnings for some tests.
I propose thus to disable $NIX_CFLAGS_COMPILE for fortran builds or to disable the format hardening for Fortran by default.