The gfortran packages cannot be used without being wrapped with
gfortran-toolchain. However, as the comment in (gnu packages
commencement) explains, there cannot be more than one gfortran-toolchain
package because of ABI issues. Thus, versions of gfortran other than the
main one are useless.
* gnu/packages/gcc.scm (gfortran-4.8, gfortran-4.9, gfortran-5,
gfortran-6, gfortran-7, gfortran-8, gfortran-9): Remove variables.
(gfortran): Define the gfortran package based on the gcc variable.
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