build: Use Fetch Content for Gtest

Use Fetch Content to get gtest and gmock

scripts/lib/meson.py

@@ -30,11 +30,6 @@ def meson_venv_bin():
 def setup(no_system_list, static_list):
     cmd = [meson_venv_bin(), "setup", build_dir]
 
-    # TODO: These special Windows exceptions should probably be in Meson
-    # or somewhere other than this script, as it's a bit hacky.
-    if env.is_windows():
-        cmd.append("-Dsystem-gtest=false")
-
     for subproject in no_system_list or []:
         cmd.append(f"-Dsystem-{subproject}=false")