Re: fixing AC_OPENMP for Apple clang

[Bruno Haible via Discussion list for the autoconf build system]

See also the ticket <https://savannah.gnu.org/support/?111055>.
There, two facts were mentioned:

* While libtool (version 2.5.3) does not support "-Xpreprocessor -fopenmp",
  it does support "-Wp,-fopenmp".

* On macOS 26.2, with cc being "Apple clang version 17.0.0 (clang-1700.6.3.2)",
  "cc -help" reports that the option '-fopenmp' exists.
  Therefore it looks like trying "-Wp,-fopenmp" is only needed for older
  versions of Apple clang (which ones?).

Bruno