Various build fixes and improvements

This pull requests contains some bugfixes for doxygen, but also cuts down the number of issues with the out-of-tree-build to one. The one issue left is that some static files are copied into / generated in `$(top_builddir)/htdocs` while many others are contained in `$(top_srcdir)/htdocs`. The solution to rename `htdocs` to `htdocs.in` and copy it to the build directory when compiling didn't seem very smart to me, but I have no other ideas yet, that's why I couldn't resolve this one issue.

You might want to look at the make rule for the ecpp files. I removed the '-p' flag which, in out-of-tree-builds, caused the components to have names beginning with '..' which further lead to no single component being callable. I didn't add the sed command that replaces the basename / -p flag functionality because all the components are directly contained in the webapp directory anyway; please add it yourself if you think it's appropriate here.