[#3698] Install gtest sources on FreeBSD

Fixes "checking for gtest source... configure: error: no gtest source at /usr/src/googletest".

Author: andrei-pavel

hammer.py

@@ -2016,7 +2016,7 @@ def link_pg_config():
             packages.extend(['cmake', 'git', 'pcre2'])
 
         if 'unittest' in features:
-            packages.append('googletest')
+            deferred_functions.append(_install_gtest_sources)
 
         install_pkgs(packages, env=env, timeout=6 * 60, check_times=check_times)