Do not use Gtest imported target (Not available in minimum CMake version required)

unitTests/CMakeLists.txt

@@ -2,10 +2,16 @@ add_executable(unit_tests mainTestRunner.cpp
     test_types.cpp
 )
 
+#TODO Use GTest::GTest once we upgrade the minimum CMake version required
 target_link_libraries(unit_tests 
     PRIVATE
         exiv2lib
-        GTest::GTest
+        ${GTEST_BOTH_LIBRARIES}
+)
+
+target_include_directories(unit_tests
+    PRIVATE
+        ${GTEST_INCLUDE_DIRS}
 )
 
 target_compile_definitions(unit_tests