always install doxygen files, even if we didn't generate them

[[Imported from SVN: r6759]]

This fixes #803, but leaves us with the ugly situation, that docs in the 
tar balls don't get installed, if doxygen is missing.

[[Imported from SVN: r6435]]

[[Imported from SVN: r6156]]

[[Imported from SVN: r6154]]

[[Imported from SVN: r6133]]

[[Imported from SVN: r5937]]

[[Imported from SVN: r5843]]

[[Imported from SVN: r5815]]

* make distcheck work
* make out-of-source builds for tarballs work
... all this only for dune-common...

[[Imported from SVN: r5811]]

[[Imported from SVN: r5804]]

[[Imported from SVN: r5793]]

- add proper web-install target that seperated recursion and local targets
- move web-install to documentation, so that you can call "make web.install" from top-src
- provide fallback targets, if doxygen and/or wml is missing

[[Imported from SVN: r5792]]

[[Imported from SVN: r5782]]

[[Imported from SVN: r5770]]

[[Imported from SVN: r5756]]

- doxygen/html wasn't properly included
- enable build only if doxygen was found

[[Imported from SVN: r5692]]

[[Imported from SVN: r5663]]

[[Imported from SVN: r5661]]

[[Imported from SVN: r5621]]

* run doxygen.css only if it exists

[[Imported from SVN: r5618]]

[[Imported from SVN: r5615]]

[[Imported from SVN: r5605]]

[[Imported from SVN: r5600]]

[[Imported from SVN: r5559]]

also rerun Doxygen when da "modules" file has changed, as used for example by dune grid do seed the module hierachy

[[Imported from SVN: r5547]]

using the wml templates from dune-web.

[[Imported from SVN: r5529]]

[[Imported from SVN: r5524]]

[[Imported from SVN: r5483]]

[[Imported from SVN: r5220]]

[[Imported from SVN: r5211]]

[[Imported from SVN: r5178]]

* use doxy-header/footer.html only wen building with dune-web
* fix #267

[[Imported from SVN: r5164]]