Fix CMake Doxygen install (match Makefile builds)

[jevinskie]

The CMake install was trying to install <gitroot>/docs/api/ instead of <buildroot>/docs/doxytemp/. Matches Makefile behavior.

[ktmf01]

Diving into this, I see now that CMake Doxygen installing is broken anyway, but this patch would make installing from a tarball even more broken.

It would probably be better to have a post build hook to rename the doxytmp/html directory to api. That is what the makefile does, so that would truely match it.

Note to self: Currently api docs are only installed when doxygen is detected, in which case they are rebuild anyway. The return here probably needs a little more sophistication.