Ensure cache checksums are deterministic

Whilst working on the Reproducible Builds[0] effort, we noticed that
fontconfig generates unreproducible cache files.

This is due to fc-cache uses the modification timestamps of each
directory in the "checksum" and "checksum_nano" members of the _FcCache
struct. This is so that it can identify which cache files are valid
and/or require regeneration.

This patch changes the behaviour of the checksum calculations to prefer
the value of the SOURCE_DATE_EPOCH[1] environment variable over the
directory's own mtime. This variable can then be exported by build
systems to ensure reproducible output.

If SOURCE_DATE_EPOCH is not set or is newer than the mtime of the
directory, the existing behaviour is unchanged.

This work was sponsored by Tails[2].

 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/specs/source-date-epoch/
 [2] https://tails.boum.org/

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