cmake: try using system installation of GTest if it's present

This is a patch that I made few months ago as Fedora forbids bundling and
using pieces of software provided by other packages in its repositories but
forgot to upstream it at that time. [1]

It has been rebased and improved so that it also reflects changes made
in #1458.

This should also make the compilation of unittests easier for our users
as they don't need to clone googletest from GitHub anymore and just use
package manager in the distro of their choice, provided that the gtest
package includes a corresponding CMake module.

[1]: https://src.fedoraproject.org/rpms/klee/blob/4c81b78/f/use-system-gtest.patch

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