build(cmake): use system GTest when available

This speeds up compilation times and makes downstream packaging easier.

CMakeLists.txt

@@ -31,6 +31,10 @@ if(NOT COMMAND add_sanitizers)
 endif()
 
 if(BUILD_TESTS)
+  find_package(GTest QUIET)
+  if(TARGET GTest::Main)
+    add_library(gtest_main ALIAS GTest::Main)
+  endif()
   if(NOT TARGET gtest_main)
     if(NOT EXISTS "${PROJECT_SOURCE_DIR}/googletest/CMakeLists.txt")
       message(FATAL_ERROR "Could not find googletest: run\n\tgit submodule update --init --recursive\nin base git checkout")