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Pkg-config files for dune modules are essentially outdated, (unless the feature is completely rewritten and fixed, as in !910).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.

added buildsystem label

requested review from @santiago.ospina

assigned to @simon.praetorius

added 44 commits

• 0bdd0e36...a59cfc87 - 42 commits from branch master
• 00ca3d7b - Configure a pkg config file only if the template file exists
• 333ab642 - create and oinstall pkg-config file only if the template exists

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resolved all threads

merged

mentioned in commit 58d72453