googletest

Use `$<INSTALL_PREFIX>` in `target_include_directories`

To make sure packages are relocatable, use `$<INSTALL_PREFIX>` in
`$<INSTALL_INTERFACE:...>` `target_include_directories`.

`$<INSTALL_PREFIX>` was introduced in CMake 2.8.11, which is already
being checked for locally.

References:

- https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#id19
- https://github.com/robotology/how-to-export-cpp-library/blob/claudio/headeronly/src/LibTemplateCMake/CMakeLists.txt#L42

Signed-off-by: Dakota Hawkins <dakotahawkins@gmail.com>