[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
 Keywords:                |       Port:
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 There were several cases of ports being broken by careless usage of
 `CC_FOR_BUILD` and/or `CXX_FOR_BUILD` while those were not set to
 anything. In result, Macports compiler settings get ignored, and a system
 default compiler is picked or even some random one. That cannot work
 correctly.
 What we actually need it that the right compiler is picked'' for the
 build.''

 Can this be fixed in the base instead of per-port patches?

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