message: cmake: do not overwrite -DCMAKE_C{XX}_COMPILER options
  Enable to configure build with GCC or any other compiler on Linux
diff: cmake_minimum_required(VERSION 3.13)
  if (UNIX)
  + if (NOT CMAKE_C_COMPILER)
  set(CMAKE_C_COMPILER "clang")
  + endif()
  + if (NOT CMAKE_CXX_COMPILER)
  set(CMAKE_CXX_COMPILER "clang++")
  endif()
  +endif()
  set(PROJECT_NAME ispc)
  set(ISPC_BUILD TRUE)
  