ribodbmaker fails if srcchk or vecscreen executables don't work

[nawrockie]

The ribodbmaker script, which is documented as only working on Linux systems, often requires a path to a working srcchk and/or vecscreen executable, from the vecscreen_plus_taxonomy package. That package has not been updated in years and there's been reports of the srcchk and vecscreen executables not working on some systems, preventing ribodbmaker from running.