cmake port not installed by cmake portgroup on builders when building clang-3.8

[Jeremy Huddleston Sequoia]

I hit a build failure on the buildbots (https://build.macports.org/builders/buildports-mavericks-x86_64/builds/14982) because the cmake port was not installed as a build dependency:

    sh: /opt/local/bin/cmake: No such file or directory

The cmake portgroup sets:

    # can use cmake or cmake-devel; default to cmake if not installed
    depends_build-append path:bin/cmake:cmake

Yet the cmake port was not installed when building clang-3.8.  It was installed when building llvm-3.8.

Does anyone see what the problem is?
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