Merge pull request #1286 from rdicosmo/add-solver-cmdline-option

Make external solver command settable via cmdline or env variable
Save version_map in cudf file as comments
Merge pull request #1289 from lefessan/save-versions-map

Save version_map in cudf file as comments
Restored lazy evaluation of external solver presence
Use the POSIX compliant command -v invocation to determine whether a command exists,
and check execution permissions if it is an external command.