message: cmake improvements for clang/libc++/libc++abi
diff: @@ -256,10 +256,18 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_FLAGS MATCHES "-stdl
  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++")
  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -stdlib=libc++")
  message(STATUS "Enoki: using libc++.")
  + else()
  + CHECK_CXX_COMPILER_AND_LINKER_FLAGS(HAS_LIBCPP_AND_CPPABI "-stdlib=libc++" "-stdlib=libc++ -lc++abi")
  + if (HAS_LIBCPP_AND_CPPABI)
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++ -D_LIBCPP_VERSION")
  + set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++ -lc++abi")
  + set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -stdlib=libc++ -lc++abi")
  + message(STATUS "Enoki: using libc++ and libc++abi.")
  else()
  message(FATAL_ERROR "When Clang is used to compile Enoki, libc++ must be available -- GCC's libstdc++ is not supported! (please insteall the libc++ development headers, provided e.g. by the packages 'libc++-dev' and 'libc++abi-dev' on Debian/Ubuntu).")
  endif()
  endif()
  +endif()
  if (ENOKI_TEST)
  