Support setting of link arguments

[awvwgk]

Currently we only support --flag arguments to set compile arguments which are also added to the link line. The --flag argument could be used to add link arguments in the hope they will be silently ignored by the compiler when not linking, but we preferably provide a command-line option to add link arguments as well rather than relying on the compiler.

See https://github.com/PIK-ICoN/NetworkDynamicsBenchmarks#compile-fortran-program for such a misuse of the --flag option from Haskell fpm.

A common usage of link arguments would would be the selection of a different sysroot in a conda-build setup.