Search system for GTest before downloading. #654

Distributions often do builds with no network access available
for both security reasons and also to ensure reproducibility.

This change tells CMake to query the system for a copy of gtest,
but if it's not available, it'll fall back to downloading via
FetchContent.

test/CMakeLists.txt

@@ -2,6 +2,8 @@ cmake_minimum_required(VERSION 3.12)
 
 include(FetchContent)
 
+find_package(GTest)
+
 enable_language(CXX)
 
 if(NOT TARGET GTest::GTest)