Support {{ compiler('mpi') }}

[isuruf]

Comment:

This way, we can setup cross compiling env things like OPAL_PREFIX and build_env compiler.
Also, we can remove hacks like https://github.com/conda-forge/conda-smithy/blob/52db47bc3064f2471c1c8927c02d1149014557fa/conda_smithy/configure_feedstock.py#L280

cc @minrk, @beckermr

[leofang]

Sounds like an alternative solution to #91 to me? (xref: conda-forge/conda-forge.github.io#1752) cc: @carterbox

[jakirkham]

cc @dalcinl

[dalcinl]

Perhaps it should be compiler('mpicc'), compiler('mpicxx'), compiler('mpifort')?
Recipes may need only one, or two, or all of them. Depending on the actual semantics of compiler('mpi'), my comment may be irrelevant. Perhaps the idea is that compiler('mpi') alone is meaningless and should error, unless something like compiler('c') is also present.

Or perhaps the compiler(...) thing could be extended such that it could be used the following way:

{{ compiler('c', 'mpi') }}
{{ compiler('cxx', 'mpi') }}
{{ compiler('fortran', 'mpi') }}

Beside these minor details about defining actual semantics, this is a great idea!