Changes

doxy: disable doc building when using doxygen 1.8.16 inside the source
tree
When we have the "images" subdirectory twice in doxygen's IMAGE_PATH
(one in @srcdir@ from prebuilt images, and one for @builddir@ for
rebuilt images). When srcdir==builddir, doxygen 1.8.16 warns that
images are
"ambiguous" (because each image is available from both paths) and just
ignores them, breaking the PDF build.
This will be fixed in the next doxygen release thanks to commit 
https://github.com/doxygen/doxygen/commit/752753c044af46cc4a22f83be8e4e1ae6be8f373
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>