change include file path

[CodingCat]

not sure why we had this change...but I found it broke the original command mvn package running in mac

@trivialfis @chenqin

[chenqin]

LGTM, I thought we had rabit import path issue with cmake file.

[trivialfis]

I'm not sure about the relative paths in public headers. We have -Idmlc-core/include, some headers in there can sneak into the translation unit replacing the one in rabit (which can lead to obscure bugs). While using rabit/serialisation.h provide some weak guarantees that no such error can occur silently.

For the reason why this could happen, include path searching starts with current file, but not limited to it. If the compiler fails to find one in relative path to current file, it will start again from paths specified by -I flag. (-I is a short hand compiler argument saying include this directory, more restricted setting would distinguish -isystem and -I, but should not be related to this PR) I would like to resolve your build issue first, it may indicates errors described above is actually happening in our code base.

[trivialfis]

Could you please provide the error message you encountered so I can take a look?

[CodingCat]

Could you please provide the error message you encountered so I can take a look?

it actually simply cannot find files like rabit/serializable.h

[hcho3]

Recent developments of Rabit have been moved into the XGBoost repository. See discussion in dmlc/xgboost#5995.