Fix linking to draco if DRACO_LIBRARIES is not defined (backport #66)

[mergify]

#57 switched to linking DRACO_LIBRARIES from DRACO_LIBRARY, but the upstream project does not define any DRACO_LIBRARIES CMake variable, see google/draco@7109fbe, while the DRACO_LIBRARIES is added as a patch by Debian distributions (see https://sources.debian.org/patches/draco/1.5.6+dfsg-3/0004-Set-DRACO_LIBRARIES-for-backwards-compatibility.patch/).

To fix compat with non-Debian distribution, we link to DRACO_LIBRARIES if defined, or draco::draco otherwise.

xref: #66

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This is an automatic backport of pull request #66 done by Mergify.

[mergify]

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