devel/kf5-extra-cmake-modules: Make qt5-help a build-dependency

[rakuco]

Summary

Make the dependency on devel/qt5-help a build-time dependency, not a build+run one.

It is only used to produce the .qch file, so it does not need to be present at runtime. This turns kf5-e-c-m into a port that has no dependency on Qt5 (or basically any other port), which makes it easier to depend on it.

Test Plan

Built other kf5 ports successfully.

[tcberner]

Looks good to me. But I faintly remember one of the other options being broken -- you might want to fix that too while your on it :D

[rakuco]

In D10234#211685, @tcberner wrote:

But I faintly remember one of the other options being broken -- you might want to fix that too while your on it :D

Don't you mean TEST? There's a comment there saying TEST=OFF is broken.