message: fix Travis clang builds
diff: @@ -6,6 +6,7 @@ before_script:
  - echo ${CXX}
  - ${CXX} --version
  - echo "CXX=$CXX" > make/local
  + - echo "CXXFLAGS+=$CXXFLAGS" >> make/local
  linux_clang: &linux_clang
  os: linux
  @@ -48,6 +49,7 @@ matrix:
  - <<: *linux_clang
  env:
  - MATRIX_EVAL="CXX=clang++-3.8"
  + CXXFLAGS="-stdlib=libc++"
  PARALLEL=2
  - <<: *linux_gcc
  env:
  @@ -56,6 +58,7 @@ matrix:
  - <<: *mpi_linux_clang
  env:
  - MATRIX_EVAL="CXX=\"mpicxx -cxx=clang++-3.8\""
  + CXXFLAGS="-stdlib=libc++"
  STAN_MPI=true
  PARALLEL=2
  - <<: *mpi_linux_gcc
  