This breaks the `clean` rule when using Ninja as the CMake
generator. Unfortunately this means `clean` doesn't try to
remove the generated documentation anymore when using Ninja.
and install them. The target for building the documentation is
`api_docs`.
This is off by default but can be enabled with the
`BUILD_DOCUMENTATION` option. The C and C++ API documentation
is always built but the Python, ".NET", and Java documentation are
only built if they are enabled in the build system. The rationale
for this is that it would be confusing to install documentation
for API bindings that are not installed.
By default `ALWAYS_BUILD_DOCS` is on which will slow down builds
significantly but will ensure that when the `install` target is
invoked the documentation is up-to-date. Unfortunately I couldn't
find a better way to do this. `ALWAYS_BUILD_DOCS` can be disabled
to get faster builds and still have the `api_docs` target available.
