do not rely on the fix in r42697 until 1.7

[Toby Peterson]

User ran into an issue with the 'metis' port because it was relying on
a fix that isn't in 1.6. Basically, if you use a custom 'configure'
target, ${configure.compiler} and related variables don't get set
properly. This requires workarounds like
http://trac.macports.org/changeset/43622

Here's the fix in base, for reference: http://trac.macports.org/changeset/42697

- Toby