use clang in build matrix 2

Author: pcmoritz

.travis.yml

@@ -80,8 +80,7 @@ matrix:
     - ARROW_TRAVIS_JAVA_BUILD_ONLY=1
     # ARROW-2999 Benchmarks are disabled in Travis CI for the time being
     # - ARROW_TRAVIS_PYTHON_BENCHMARKS=1
-    - CC="clang-6.0"
-    - CXX="clang++-6.0"
+    - MATRIX_EVAL="CC=clang-6.0 && CXX=clang++-6.0"
     before_script:
     # (ARROW_CI_CPP_AFFECTED implies ARROW_CI_PYTHON_AFFECTED)
     - if [ $ARROW_CI_PYTHON_AFFECTED != "1" ]; then exit; fi

python/CMakeLists.txt

@@ -137,6 +137,7 @@ if ("${COMPILER_FAMILY}" STREQUAL "clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-parentheses-equality")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-constant-logical-operand")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-missing-declarations")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-sometimes-uninitialized")
 
   # We have public Cython APIs which return C++ types, which are in an extern
   # "C" blog (no symbol mangling) and clang doesn't like this