[MacPorts] #69839: Handle CC_FOR_BUILD and CXX_FOR_BUILD where they are used but not actually set

[MacPorts]

#69839: Handle CC_FOR_BUILD and CXX_FOR_BUILD where they are used but not actually
set
---------------------------+--------------------
  Reporter:  barracuda156  |      Owner:  (none)
      Type:  enhancement   |     Status:  new
  Priority:  Normal        |  Milestone:
 Component:  base          |    Version:  2.9.3
Resolution:                |   Keywords:
      Port:                |
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Comment (by ryandesign):

 That could at least be handled by the meson portgroup. That prompted me to
 check the portgroups. At the moment, only the muniversal 1.1 portgroup
 sets them. It also deals with `CFLAGS_FOR_BUILD`, `CPPFLAGS_FOR_BUILD`,
 `CXXFLAGS_FOR_BUILD`, and `LDFLAGS_FOR_BUILD`. I'm not sure who uses those
 or what we should put in there by default, if we added support for those
 to base as well.

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Ticket URL: <https://trac.macports.org/ticket/69839#comment:4>
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