Support more configurability in GN toolchains

tools/gn/example/build/toolchain/BUILD.gn

+# Copyright 2014 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.
+
+toolchain("gcc") {
+  tool("cc") {
+    depfile = "{{output}}.d"
+    command = "gcc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    depsformat = "gcc"
+    description = "CC {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+    ]
+  }
+
+  tool("cxx") {
+    depfile = "{{output}}.d"
+    command = "g++ -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
+    depsformat = "gcc"
+    description = "CXX {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+    ]
+  }
+
+  tool("alink") {
+    rspfile = "{{output}}.rsp"
+    command = "rm -f {{output}} && ar rcs {{output}} @$rspfile"
+    description = "AR {{target_output_name}}{{output_extension}}"
+    rspfile_content = "{{inputs}}"
+    outputs = [
+      "{{target_out_dir}}/{{target_output_name}}{{output_extension}}"
+    ]
+    default_output_extension = ".a"
+    output_prefix = "lib"
+  }
+
+  tool("solink") {
+    soname = "{{target_output_name}}{{output_extension}}"  # e.g. "libfoo.so".
+    rspfile = soname + ".rsp"
+
+    command = "g++ -shared {{ldflags}} -o $soname -Wl,-soname=$soname @$rspfile"
+    rspfile_content = "-Wl,--whole-archive {{inputs}} {{solibs}} -Wl,--no-whole-archive {{libs}}"
+
+    description = "SOLINK $soname"
+
+    # Use this for {{output_extension}} expansions unless a target manually
+    # overrides it (in which case {{output_extension}} will be what the target
+    # specifies).
+    default_output_extension = ".so"
+
+    outputs = [
+      soname,
+    ]
+    link_output = soname
+    depend_output = soname
+    output_prefix = "lib"
+  }
+
+  tool("link") {
+    outfile = "{{target_output_name}}{{output_extension}}"
+    rspfile = "$outfile.rsp"
+    command = "g++ {{ldflags}} -o $outfile -Wl,--start-group @$rspfile {{solibs}} -Wl,--end-group {{libs}}"
+    description = "LINK $outfile"
+    rspfile_content = "{{inputs}}"
+    outputs = [ outfile ]
+  }
+
+  tool("stamp") {
+    command = "touch {{output}}"
+    description = "STAMP {{output}}"
+  }
+
+  tool("copy") {
+    command = "cp -af {{source}} {{output}}"
+    description = "COPY {{source}} {{output}}"
+  }
+}