With a hacky workaround, because the generate_gir() implementation
has a bug and does not pass the file dependencies properly down
to the custom_target() that it creates internally
Let g-ir-scanner parse the original headers for the C signatures
and just add the gtk-doc comments in the generated wp-gtkdoc.h file
Also cleanup a bit gen-api-gtkdoc.py and make it omit printing most of the
whitespace clutter that it used to print inside the gtk-doc comments
* Use custom_target() instead of configured shell scripts
* Do not copy all the .rst files in the build directory
* Setup dependencies between targets
* Tidy up dependencies lookup
* Remove unused files
* Upgrade Doxyfile to the latest version and cleanup used options
