[C++] clang-7 matrix entry is build using gcc

[asfimport]

Travis sets the following environment variables:

$ export CC="clang-7"
$ export CXX="clang++-7"
$ export TRAVIS_COMPILER=g++
$ export CXX=g++
$ export CXX_FOR_BUILD=g++
$ export CC=gcc
$ export CC_FOR_BUILD=gcc
$ export PATH=/usr/lib/ccache:$PATH

Reporter: Uwe Korn / @xhochy
Assignee: Francois Saint-Jacques / @fsaintjacques

PRs and other links:

• GitHub Pull Request #3769

_{Note: This issue was originally created as ARROW-4672. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3769
#3769