doc: fix out-of-tree build

When building outside of the source tree, the Doxyfile needs to be
generated in the build tree and should point to the proper paths for
include directories and html footer.

The generated api files to install should also be taken from the build
tree instead of the source tree.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
4 files changed