fix(gtest): prefer target_link_libraries
fix(gtest): prefer target_link_libraries

From commit 880896c6f4814f4c7798355a652dc6167be2b75f in googletest cmake
test configuration fails with errors like

CMake Error: install(EXPORT "gtestConfigInternal" ...) includes target
  "gtest" which requires target "i3ipc++" that is not in the export set.

The issue was that link_libraries also affected the googletest internal
targets that came after it making the whole thing fail.

This could also have been solved by moving link_libraries after the
configuration of googletest but according to the cmake documentation [1]
target_link_libraries should be prefered over link_libraries anyways.

[1]: https://cmake.org/cmake/help/latest/command/link_libraries.html

Fixes #1393

jaagr/polybar,jaagr/polybar,jaagr/lemonbuddy,jaagr/lemonbuddy,jaagr/lemonbuddy,jaagr/polybar