Add notice about how to export unofficial CMake targets.

[BillyONeal]

Adds documentation about how to add CMake targets when upstream does not provide such exported targets.

[strega-nil-ms]

the wording could probably be better from me as well, but this is my first try

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dg0yt]

This effectively puts the unofficial targets into the reported usage. So sort-of still promoting the lock-in even if alternatives exists (Find module, pkg-config module).

Probably there are port which lack the ::<port>:: namespace part. This could be a post-install check.

[dg0yt]

In some cases it is necessary to export private API (Qt, poppler). There might be a hint how to use EXPORT_NAME to modify exported names.

vcpkg/ports/poppler/export-unofficial-poppler.patch

Line 15 in f5d0ae2

+set_target_properties(poppler PROPERTIES EXPORT_NAME poppler-private)

[BillyONeal]

This effectively puts the unofficial targets into the reported usage. So sort-of still promoting the lock-in even if alternatives exists (Find module, pkg-config module).

This whole section is "if you are adding CMake bindings" which we generally only do if there is no such thing present.

Probably there are port which lack the ::<port>:: namespace part. This could be a post-install check.

Probably not worth breaking anything already in the catalog. We added these docs primarily so that we have something to link to on what we want when people ask.