Support out-of-tree builds by not hardcoding paths
Improve Doxygen support in Autotools build system

* Add `m4/geany-doxygen.m4` file
* Support `DOXYGEN` environment variable setting which Doxygen to use
* Support `--enable-api-docs` to activate building Doxygen docs
    (off by default)
Use improved Doxygen Autotools support