Missing `abseil-cpp` targets

Extension building fails with Ninja generator. The issue is how Ninja handles extra dependencies which are populated by `ExternalProject_Add`. I guess that `abseil-cpp` targets should somehow specified (maybe `BUILD_BYPRODUCTS`?). There is no issues with Unix Makefiles generator.

```shell
$ CMAKE_GENERATOR=Ninja python setup.py build_ext
...
ninja: error: '.../tree-0.1.8/tree/abseil-cpp/lib/libabsl_int128.a', needed by '.../tree-0.1.8/build/lib.linux-x86_64-cpython-310/tree/_tree.cpython-310-x86_64-linux-gnu.so', missing and no known rule to make it
Traceback (most recent call last):
...
subprocess.CalledProcessError: Command '['cmake', '--build', '.', '-j8', '--config', 'Release']' returned non-zero exit status 1.
```