doc: Fix Doxygen include for srcdir != builddir

doc: Fix Doxygen include for srcdir != builddir

Instead of giving Doxygen a series of absolute paths to the source
files and a relative path to the output directory, run it from the
source directory with purely relative paths to the source files, and
give it an absolute path to the build directory.

This fixes the parsing of README.md with a separate build directory,
since the relative includes for doc/quick-guide.md and doc/compat.md
don't resolve otherwise. Doxygen's INCLUDE_PATH turns out not to fix
this either, since that's just a set of paths to open and parse, rather
than an analogue to cpp's -I.

Signed-off-by: Daniel Stone <daniels@collabora.com>