GN: support compiler_prefix, use it for ccache on bots.

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++"
 
   if (is_android) {
     ar = "$ndk/toolchains/$ndk_gccdir-4.9/prebuilt/$ndk_host/$ndk_target/bin/ar"
     cc = "$ndk/toolchains/llvm/prebuilt/$ndk_host/bin/clang"
     cxx = "$ndk/toolchains/llvm/prebuilt/$ndk_host/bin/clang++"
   }
 
   extra_cflags = ""
   extra_cflags_c = ""
   extra_cflags_cc = ""
+
+  compiler_prefix = ""
 }
 
 config("no_rtti") {
   cflags_cc = [ "-fno-rtti" ]
 }
 
 config("default") {
   cflags = [
     "-g",
     "-fstrict-aliasing",
@@ skipping 88 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}} $extra_cflags $extra_cflags_c -c {{source}} -o {{output}}"
+    command = "$compiler_prefix $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 ... $extra_cflags $extra_cflags_c -o {{output}}"
+    description =
+        "$compiler_prefix $cc ... $extra_cflags $extra_cflags_c -o {{output}}"
   }
 
   tool("cxx") {
     depfile = "{{output}}.d"
-    command = "$cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} $extra_cflags $extra_cflags_cc -c {{source}} -o {{output}}"
+    command = "$compiler_prefix $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 ... $extra_cflags $extra_cflags_cc -o {{output}}"
+    description =
+        "$compiler_prefix $cxx ... $extra_cflags $extra_cflags_cc -o {{output}}"
   }
 
   tool("asm") {
     depfile = "{{output}}.d"
-    command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{asmflags}} -c {{source}} -o {{output}}"
+    command = "$compiler_prefix $cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{asmflags}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
-    description = "$cc ... -o {{output}}"
+    description = "$compiler_prefix $cc ... -o {{output}}"
   }
 
   tool("alink") {
     command = "rm -f {{output}} && $ar rcs {{output}} {{inputs}}"
     outputs = [
       "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
     ]
     default_output_extension = ".a"
     output_prefix = "lib"
     description = "$ar {{output}} ..."
   }
 
   tool("solink") {
     soname = "{{target_output_name}}{{output_extension}}"
 
     rpath = "-Wl,-soname,$soname"
     if (is_mac) {
       rpath = "-Wl,-install_name,@rpath/$soname"
     }
 
-    command = "$cxx -shared {{ldflags}} {{inputs}} {{solibs}} {{libs}} $rpath -o {{output}}"
+    command = "$compiler_prefix $cxx -shared {{ldflags}} {{inputs}} {{solibs}} {{libs}} $rpath -o {{output}}"
     outputs = [
       "{{root_out_dir}}/$soname",
     ]
     output_prefix = "lib"
     default_output_extension = ".so"
-    description = "$cxx -shared ... -o {{output}}"
+    description = "$compiler_prefix $cxx -shared ... -o {{output}}"
     pool = ":link_pool"
   }
 
   tool("link") {
-    command = "$cxx {{ldflags}} {{inputs}} {{solibs}} {{libs}} -o {{output}}"
+    command = "$compiler_prefix $cxx {{ldflags}} {{inputs}} {{solibs}} {{libs}} -o {{output}}"
     outputs = [
       "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
     ]
-    description = "$cxx ... -o {{output}}"
+    description = "$compiler_prefix $cxx ... -o {{output}}"
     pool = ":link_pool"
   }
 
   tool("stamp") {
     command = "touch {{output}}"
   }
 
   tool("copy") {
     command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
   }
 }