[ANN] building ODK documentation now requires doxygen 1.8.4

dear all,

have just merged branch feature/doxygen to master (for the 4.2 release
later this year): the ODK IDL documentation is now built with doxygen,
and the legacy autodoc tool is removed.

this means that doxygen 1.8.4 is required, because older versions do not
support UNO IDL.  configure checks for the required version.

if you don't want to build or download latest doxygen, just use
--disable-doxygen; it is only really required for release builds.

building doxygen is very easy on Linux, Windows (cygwin) and MacOSX,
just clone the git repo and build:

> git clone git://github.com/doxygen/doxygen.git
> cd doxygen
> ./configure
> make -j8

then add a line like this to autogen.input:

 --with-doxygen=/home/foo/doxygen/bin/doxygen

there are also binary downloads on the doxygen site which i haven't
tried but i guess they'd work too:

http://www.stack.nl/~dimitri/doxygen/download.html

PS: there are still quite a bunch of doxygen warnings about unresolved
links and such in the IDL files, patches appreciated to clean those up.

PPS: due to limitations in MSI tooling the documentation generated on
Windows is not suitable for publishing on the website; build it on a
real UNIXy OS for that purpose