Using SPM_USE_EXTERNAL_ABSL

[ryandesign]

I tried to use -DSPM_USE_EXTERNAL_ABSL=ON when running cmake for sentencepiece 0.1.99. The error is:

CMake Error at CMakeLists.txt:150 (add_subdirectory):
  add_subdirectory given source "third_party/abseil-cpp" which is not an
  existing directory.

There is no directory abseil-cpp in the third_party directory.

I have abseil 20230125.3 installed in my prefix; .cmake and .pc files are there for finding it.

I searched your project for SPM_USE_EXTERNAL_ABSL and found only hits in the source code; no hits in documentation explaining how to use it.

Searching the project for -DSPM_USE_EXTERNAL_ABSL turned up #576 which explains how to use it (manually cloning abseil-cpp into the third_party directory). This should be in documentation included with the project and perhaps cmake could direct the user to that documentation if they use this flag without having cloned into that directory.

You should support using an installed abseil, not just one manually cloned into the third_party directory.

[Halmoni100]

See #920

[taku910]

Fixed in v0.2.0