Unset cached variables if necessary after CMake failure

[pgavlin]

A number of tests done while configuring with CMake insert configuration results into the CMake cache, which makes configuration failures a bit unfriendly to fix (as the cached results must often be removed in order to retrigger the config tests). We should consider unsetting cached variables that cause configuration failures s.t. reconfiguring reruns the requisite tests.

See dotnet/coreclr#313 for an example.

[MattWhilden]

@pgavlin I'll presume you don't have the bandwidth to close down on this. I'll mark it as open for the community so feel free to dump any other thoughts in here.

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