The cmake option BUILD_SHARED_LIBS is a cmake built-in option
to control if libraries are by default SHARED or STATIC.
For large projects, it is desired to be able to design gtsam as a
shared or static library regardless of the value of BUILD_SHARED_LIBS.
This change is unobtrusive, two new cmake options are created, to
force gtsam to be a shared or static library. If neither option is
set (this is the default), the behavior of gtsam remains unchanged
which is to use BUILD_SHARED_LIBS decision.
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