This commit differentiate linux from other platforms by
using libc compatibility as a criterion for deciding
which buildcaches / binaries can be reused. Other
platforms still use OS compatibility.
On linux a libc is injected by all compilers as an implicit
external, and the compatibility criterion is that a libc is
compatible with all other libcs with the same name and a
version that is lesser or equal.
Some concretization unit tests use libc when run on linux.
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