GN: add extra_cflags et al.

gn/BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   ar = "ar"
   cc = "cc"
   cxx = "c++"
+
+  extra_cflags = ""
+  extra_cflags_c = ""
+  extra_cflags_cc = ""
 }
 
 config("no_rtti") {
   cflags_cc = [ "-fno-rtti" ]
 }
 
 config("default") {
   cflags = [
     "-g",
     "-fstrict-aliasing",
@@ skipping 37 matching lines @@
 pool("link_pool") {
   depth = 0  #unlimited
 }
 
 toolchain("gcc_like") {
   lib_switch = "-l"
   lib_dir_switch = "-L"
 
   tool("cc") {
     depfile = "{{output}}.d"
-    command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} $extra_cflags $extra_cflags_c -c {{source}} -o {{output}}"
     depsformat = "gcc"
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
     description = "$cc ... -o {{output}}"
   }
 
   tool("cxx") {
     depfile = "{{output}}.d"
-    command = "$cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
+    command = "$cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} $extra_cflags $extra_cflags_cc -c {{source}} -o {{output}}"
     depsformat = "gcc"
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
     description = "$cxx ... -o {{output}}"
   }
 
   tool("asm") {
     depfile = "{{output}}.d"
     command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{asmflags}} -c {{source}} -o {{output}}"
@@ skipping 42 matching lines @@
   }
 
   tool("stamp") {
     command = "touch {{output}}"
   }
 
   tool("copy") {
     command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
   }
 }