CMake "webpage" target does not fail when it should
docs/doxygen/CMakeLists.txt creates a "webpage" target in all cases, but if the webpage build is not possible, the target still results successfully, which prevents it from being useful as a dependency and can be confusing in scripted use.
admin/builds/documentation.py uses the result of the "webpage" build target to determine whether to proceed with link checking. Since the mock "webpage" target does not cause a failure, Jenkins failures are harder to diagnose.
If it is preferable to have the output of "webpage" as a build target instead of only at configure time, the target should at least cause a failure.