[MacPorts] #35360: add portgroup for mpi compilers

[MacPorts]

#35360: add portgroup for mpi compilers
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 Reporter:  mmoll@…               |       Owner:  macports-tickets@…                   
     Type:  enhancement           |      Status:  new                                  
 Priority:  Normal                |   Milestone:                                       
Component:  ports                 |     Version:  2.1.2                                
 Keywords:  mpi, openmpi, mpich2  |        Port:                                       
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 The MPI-related variant code blocks that many ports use for either
 enabling MPI support or allowing users to switch between MPI
 implementations are more or less all the same. It'd be nice to put all
 that in a PortGroup. Sean Farley (cc-ed) has already done that:
 https://bitbucket.org/seanfarley/scienceports/src/5b57adccee2f/_resources/port1.0/group/mpi-1.0.tcl
 It looks reasonable to me. Can one of the MacPort gurus have a look at
 this and see if this could have some undesirable side-effects?

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Ticket URL: <https://trac.macports.org/ticket/35360>
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