libpolyml-dev is missing files

[Phil Clayton]

There are issues with two files:

1. The file polyml.pc is not provided by any Poly/ML package. Without this, pkg-config cannot be used to create the arguments for linking against libpolyml: the command `pkg-config --libs polyml` fails instead of producing ld flags. This issue was identified by a user in the following report:
https://github.com/giraffelibrary/giraffe/issues/2#issuecomment-940525255
(second half, relating to Poly/ML).

2. The file polyc is provided in the package polyml but the utility cannot work without one of the files libpolyml.so or libpolyml.a provided in libpolyml-dev. libpolyml-dev is not a required by polyml so polyc should be provided by libpolyml-dev. Otherwise, package installations appear broken, as reported by a user in the following report:
https://github.com/polyml/polyml/issues/161

Note that the current package version, 5.7.1, was released nearly 4 years ago at the time of reporting this bug:
https://github.com/polyml/polyml/releases/tag/v5.7.1
If this version is updated, it should be noted that the latest version, 5.8.2, no longer depends on libffi. (Version 5.8.1 was the last version to depend on libffi.)