script-based build: add extfld and exfield

[michaellass]

Using the build scripts compile.make, library.make and link.make currently fails because extfld.f and exfield.f are not built and not included in the library. This PR includes adds both files to the corresponding scripts.

[jayponder]

Hi, Thanks. I'll fix this outside of your pull request, as there are likely several other places where this kind of change needs to be made. And I would note that these build scripts are really obsolete at this point, and we recommend use of the Makefiles, which should be correct and up to date.

[michaellass]

I'll close this as it has been open for nearly a year without progress.