Issue with Intel MPI detection

[RemiLacroix-IDRIS]

Hello,

It feels like there is a bug in Meson when trying to detect Intel MPI.

The implemented strategy is to run the MPI compiler wrapper with the -show option which is fine but the way the wrapper is chosen seems wrong:

meson/mesonbuild/dependencies/mpi.py

Lines 67 to 72 in 471c05d

	if language == 'c':
	tool_names = [os.environ.get('I_MPI_CC'), 'mpiicc']
	elif language == 'cpp':
	tool_names = [os.environ.get('I_MPI_CXX'), 'mpiicpc']
	elif language == 'fortran':
	tool_names = [os.environ.get('I_MPI_F90'), 'mpiifort']

Meson tries to get the wrapper path or name from the I_MPI_CC, I_MPI_CXX and I_MPI_FORT environment values which makes no sense since according to Intel MPI documentation those should be used to "set the path/name of the underlying compiler to be used [by the wrapper]" (https://www.intel.com/content/www/us/en/develop/documentation/mpi-developer-reference-windows/top/environment-variable-reference/compilation-environment-variables.html).

That means that if I_MPI_CC, I_MPI_CXX and I_MPI_FORT are correctly set, Meson will try the -show option on the actual compiler instead of the wrapper and thus will fail to detect Intel MPI.

I would suggest using the same environment variable as the non-Intel MPI code path. I can provide a PR.

Best regards,
Rémi