Fix bug not building apidoc and qch file by default, if enabled

For easier troubleshooting, a status line is now displayed if
qhelpgenerator was found.

A new cmake option BUILD_APIDOC has been added (default:on), which
can be used to disable the creation of api doc if this is not desired.

BUG:419202
FIXED-IN:2.30.90 (KDE releases 20.03.90)