don't validate compiler flags provided with CMAKE_TUNE_FLAGS anymore

set(CMAKE_TUNE_FLAGS "${CMAKE_TUNE_DEFAULT}" CACHE STRING "Compiler specific optimization or instrumentation")

separate_arguments(CMAKE_TUNE_FLAGS)

include(CheckCXXCompilerFlag)

foreach(_FLAG ${CMAKE_TUNE_FLAGS})

  string(REGEX REPLACE "[=\"]" "" _FLAGX ${_FLAG})

  check_cxx_compiler_flag("${_FLAG}" COMPILER_SUPPORTS${_FLAGX})

  if(COMPILER_SUPPORTS${_FLAGX})

  target_compile_options(lammps PRIVATE ${_FLAG})

  else()

    message(WARNING "${_FLAG} found in CMAKE_TUNE_FLAGS, but not supported by the compiler, skipping")

  endif()

endforeach()

########################################################################

# Basic system tests (standard libraries, headers, functions, types)   #