cmake: Fix mac build
cmake: Fix mac build

Clang @ Apple LLVM version 9.0.0 (clang-900.0.39.2) fails to
link when SOVERSION is set for bundles. Don't set it on Apple
targets.

google/draco,louquillio/draco,google/draco,louquillio/draco,louquillio/draco,louquillio/draco,louquillio/draco,google/draco,louquillio/draco,google/draco,google/draco,google/draco,google/draco