Allow the bootstrap binary of GN (the one in TEMPDIR) to build and run on FreeBSD.

tools/gn/settings.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/settings.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 
@@ skipping 19 matching lines @@
   // one-off data without doing generation.
   if (!toolchain_output_dir_.is_null())
     toolchain_gen_dir_ = SourceDir(toolchain_output_dir_.value() + "gen/");
 
 #if defined(OS_WIN)
   target_os_ = WIN;
 #elif defined(OS_MACOSX)
   target_os_ = MAC;
 #elif defined(OS_LINUX)
   target_os_ = LINUX;
+#elif defined(OS_FREEBSD)
+  target_os_ = FREEBSD;
 #else
   #error implement me
 #endif
 }
 
 Settings::~Settings() {
 }