[C++] Clang Travis build (matrix entry 2) might not actually be using clang

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See for example https://travis-ci.org/apache/arrow/jobs/448267169:

Setting environment variables from .travis.yml
$ export ANACONDA_TOKEN=[secure]
$ export ARROW_TRAVIS_USE_TOOLCHAIN=1
$ export ARROW_TRAVIS_VALGRIND=1
$ export ARROW_TRAVIS_PLASMA=1
$ export ARROW_TRAVIS_ORC=1
$ export ARROW_TRAVIS_COVERAGE=1
$ export ARROW_TRAVIS_PARQUET=1
$ export ARROW_TRAVIS_PYTHON_DOCS=1
$ export ARROW_BUILD_WARNING_LEVEL=CHECKIN
$ export ARROW_TRAVIS_PYTHON_JVM=1
$ export ARROW_TRAVIS_JAVA_BUILD_ONLY=1
$ export CC="clang-6.0"
$ export CXX="clang++-6.0"
$ export TRAVIS_COMPILER=gcc
$ export CXX=g++
$ export CC=gcc
$ export PATH=/usr/lib/ccache:$PATH
cache.1
Setting up build cache

The CC and CXX command line variables are overwritten by travis (because the travis toolchain is set to gcc).

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

PRs and other links:

• GitHub Pull Request #2878

_{Note: This issue was originally created as ARROW-3659. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 2878
#2878