[cmake] make velocypack an imported library

[torbjoernk]

On systems with velocypack installed in a non-default location, the include directory for velocypack as detected by the FindVelocypack.cmake module was not added to the include path for the fuerte library target.

With this change the FindVelocypack.cmake module provides an imported library target as recommended by modern CMake providing all required information to link against it including include path.

[OmarAyo]

Hi @torbjoernk

Many thanks for your improvements. We are using an Apache 2 CLA for ArangoDB and its companion projects, which can be found here: https://www.arangodb.com/documents/cla.pdf In order to accept your changes may you just fill in the form, sign and send a scanned copy over to cla[at]arangodb[.]com or via fax +49-221-2722999-88

[ObiWahn]

Very nice change!