#8 add --enable-doc option

[Thomas Klausner]

The attached patch (by Niclas Rosenvik <nros>) adds a configure --enable-doc option. If it is not provided, the configure script does not look for doxygen.</nros>

Rationale: doxygen is a big package, and for packaging it's nice to be able to disable looking for it, so it's not used automatically if it's installed, but by choice.

Of course, you could solve this differently, the patch is just one option.