use libc++ instead of libstdc++ with Clang on Linux

to fix this error:

/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/cstddef:51:11:
error: no member named 'max_align_t' in the global namespace using ::max_align_t;

related:

catchorg/Catch2#334

ci/build-linux.sh

@@ -13,6 +13,7 @@ else
  # Assume CXX = clang
  export CC=/usr/bin/clang-3.6
  export CXX=/usr/bin/clang-3.6
+ export CXXFLAGS="-stdlib=libc++"
  export LDFLAGS="-fdefine-sized-deallocation"
 fi
 export F77=/usr/bin/gfortran-$GCC_VERSION