Bug#799543: dh_doxygen: fails if no docs found, even in binary-only builds

["Aaron M. Ucko" <ucko@debian.org>]

Package: doxygen
Version: 1.8.9.1-5
Severity: important

Builds of signond covering only its architecture-dependent binary
packages (as on the autobuilders) have been failing:

  dh_doxygen
  dh_doxygen: Doxygen documentation not found
  make[2]: *** [override_dh_install] Error 2

I presume this error represents an intended sanity check, which could
be helpful in some circumstances.  However, the typical dh_*
convention is to treat having nothing to do as a (trivial) success
rather than a failure.  This convention is particularly appropriate
here, because many source packages (signond included) split
documentation into architecture-independent binary packages that not
all builds cover.

As such, please either downgrade this error to a warning or suppress
it altogether, at least in the case of builds covering only a subset
of possible binary packages.

See also sphinx-common bug https://bugs.debian.org/745690.

Thanks!