Configure a pkg config file only if the template file exists

Pkg-config files for dune modules are essentially outdated, (unless the feature is completely rewritten and fixed, as in !910). If you create a simple application module that does not even get consumed by or is a dependency of other modules, it doesn't make much sense to create such .pc files. Unfortunately, the corresponding template is still required for cmake to succeed. This MR fixes this and only generates a pkg config file if the associated template .pc.in exists.