update macOS gcc@5 installation guide (#3003)

After installing ``gcc@5``, ``CMAKE_C_COMPILER`` will not be set to gcc-5 in some macOS environment automatically and the installation of xgboost will still fail. Manually setting the compiler will solve the problem.
This commit is contained in:
Zhirui Wang 2018-01-05 03:28:26 +08:00 committed by Tianqi Chen
parent 14c6392381
commit bf43671841

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@ -26,6 +26,11 @@ Please install ``gcc@5`` from `Homebrew <https://brew.sh/>`_::
brew install gcc@5
After installing ``gcc@5``, set it as your compiler::
export CC = gcc-5
export CXX = g++-5
Linux
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