[MacPorts] #53348: OpenBLAS*: redo compiler variants

[MacPorts]

#53348: OpenBLAS*: redo compiler variants
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 Reporter:  michaelld    |      Owner:  NicosPavlov
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  OpenBlas, OpenBlas-devel
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 Problem: the compiler variants for OpenBLAS* ports do not work as robustly
 as they might. For example, just because +clang is used for the C/C++
 compiler should not mean that gcc5 should be require for Fortran -- that
 should be a variant of the user's choice with a default provided. Further,
 declaring each +gcc* variant independently of the others results in code
 bloat and is challenging to maintain. [Ideally we would find a way to use
 the compilers PortGroup, but I don't understand it well enough to use it.]

 Solution: Move to more of what the compilers PortGroup does: a list of GCC
 variants, then do "for loops" to declare variants and do variant checking.
 Allow for +clang and +gcc* at the same time, or just +gcc*; the former
 uses Clang for C/C++ and GCC* for Fortran while the latter uses GCC* for
 the whole compiler suite. This setup makes it simple to add and remove GCC
 versions as they become available or obsolete.

 I'm attaching a patch for this change. As I'm a co-maintainer, this ticket
 is mostly courtesy for MP developers and the other co-maintainer to
 provide feedback as to whether this is wise and/or done well.

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Ticket URL: <https://trac.macports.org/ticket/53348>
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