doxygen: do not depend on install

In Fedora's RPM and probably other distros/build systems as well the build process consist of:
1) unpacking sources/patching/preparation of sources
2) build
3) installation to buildroot
4) packaging of the buildroot

The current implementation doesn't allow to do the following (e.g. with Lunchbox project):
@build stage
make
make doxygen
@install stage
make install DESTDIR=$buildroot

This fails on 'make doxygen' trying to create /usr/share directories, which is something it shoudn't do. It should just create the docs. This patch makes the above to work.