Doxygen tag files not written during out-of-source build

qdoc is supposed to write Doxygen-compatible tag files (e.g. qtcore.tags) as part of the build process. All project files in the Qt source use a relative path such as ../../../doc/$module/$module.tags. When doing an out-of-source build the tag files are never written and there's no indication why anything would've failed. The reason is that the function responsible for opening the tag file for writing, QDocTagFiles::generateTagFile, silently returns when the file can't be opened. In addition the tag file path is relative to the source directory, not the build directory, and the referenced directory does not exist.

This means there are two separate, albeit connected, issues:

• Non-existent error handling in QDocTagFiles::generateTagFile makes diagnosis much harder
• Tag file path in qdocconf files should be relative to build directory