Override clang++ name on Linux only

.travis.yml

@@ -7,7 +7,7 @@ install:
 # /usr/bin/gcc is 4.6 always, but gcc-X.Y is available.
 - if [ "$CXX" = "g++" ]; then export CXX="g++-4.9" CC="gcc-4.9"; fi
 # /usr/bin/clang is 3.4, lets override with modern one.
-- if [ "$CXX" = "clang++" ]; then export CXX="clang++-3.7" CC="clang-3.7"; fi
+- if [ "$CXX" = "clang++" ] && [ "$TRAVIS_OS_NAME" = "linux" ]; then export CXX="clang++-3.7" CC="clang-3.7"; fi
 - echo ${PATH}
 - echo ${CXX}
 - ${CXX} --version