[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 kencu):

 meson, in particular,  uses CC_FOR_BUILD and CXX_FOR_BUILD when cross-
 compiling.

 https://mesonbuild.com/Cross-compilation.html

 https://mesonbuild.com/Reference-tables.html#environment-variables-per-
 machine

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Ticket URL: <https://trac.macports.org/ticket/69839#comment:3>
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