modernize CMake script code and add version tests for Clang and Intel to...

    message(FATAL_ERROR "Cannot find the 'omp.h' header file required for full OpenMP support")

  endif()

  if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 8.99.9))

    # GCC 9.x strictly implements OpenMP 4.0 semantics for consts.

    add_definitions(-DLAMMPS_OMP_COMPAT=4)

  if (((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 8.99.9)) OR

      ((CMAKE_CXX_COMPILER_ID STREQUAL "Clang") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 9.99.9)) OR

      ((CMAKE_CXX_COMPILER_ID STREQUAL "Intel") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 18.99.9))

      )

    # GCC 9.x and later plus Clang 10.x and later implement strict OpenMP 4.0 semantics for consts.

    # Intel 18.0 was tested to support both, so we switch to OpenMP 4+ from 19.x onward to be safe.

    target_compile_definitions(lammps PRIVATE -DLAMMPS_OMP_COMPAT=4)

  else()

    add_definitions(-DLAMMPS_OMP_COMPAT=3)

    target_compile_definitions(lammps PRIVATE -DLAMMPS_OMP_COMPAT=3)

  endif()

  target_link_libraries(lammps PRIVATE OpenMP::OpenMP_CXX)

endif()

//

// To date, most compilers still accept the OpenMP 3.0 form,

// so this is what LAMMPS primarily uses.  For those compilers

// that strictly implement OpenMP 4.0 (such as GCC 9.0), we

// give up default(none).

// that strictly implement OpenMP 4.0 (such as GCC 9.0 and later

// or Clang 10.0 and later), we give up default(none).

#if LAMMPS_OMP_COMPAT == 4

#    define LMP_SHARED(...)

#    define LMP_DEFAULT_NONE default(shared)