Hide the use of FetchContent behind a flag

FetchContent is now disabled by default, when the project is included as
an subproject by another top-level project.
Also, every usage of FetchContent is behind a separate flag to enable and
disable the usage with granular control.

tests/CMakeLists.txt

@@ -2,12 +2,22 @@ include(GoogleTest)
 
 set(DRAMSYS_TEST_RESOURCES_DIR "${CMAKE_CURRENT_SOURCE_DIR}/resources")
 
-FetchContent_Declare(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest
-    GIT_TAG release-1.12.1)
+option(DRAMSYS_USE_FETCH_CONTENT_GTEST "Enable FetchContent to provide gtest" ${DRAMSYS_USE_FETCH_CONTENT})
 
-FetchContent_MakeAvailable(googletest)
+if (DRAMSYS_USE_FETCH_CONTENT)
+    if (DRAMSYS_USE_FETCH_CONTENT_GTEST)
+        FetchContent_Declare(
+            GTest
+            GIT_REPOSITORY https://github.com/google/googletest
+            GIT_TAG release-1.12.1
+            OVERRIDE_FIND_PACKAGE
+        )
+
+        FetchContent_MakeAvailable(GTest)
+    endif()
+endif()
+
+find_package(GTest REQUIRED)
 
 add_subdirectory(tests_configuration)
 add_subdirectory(tests_dramsys)