find_package_by_config: do not add subdirs on commandline.

We don't need -I/usr/include/Qsci and the like.
In find_package_by_files, return undef upon failure.

That way callers (tests for qscintilla and aspell) do write an error
message when the files are not found, instead of silently proceeeding.
Allow using the system wide docbook-xsl installation