Add an output_dir override to GN.

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 4c5fae1adf63926b3f02bc244dad09f6dfe8e08c..46cd57edabfe3219efa2cee8bd9f323b812ccd48 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -1119,6 +1119,34 @@ const char kOutputExtension_Help[] =
     "    }\n"
     "  }\n";
 
+const char kOutputDir[] = "output_dir";
+const char kOutputDir_HelpShort[] =
+    "output_dir: [directory] Directory to put output file in.";
+const char kOutputDir_Help[] =
+    "output_dir: [directory] Directory to put output file in.\n"
+    "\n"
+    "  For library and executable targets, overrides the directory for the\n"
+    "  final output. This must be in the root_build_dir or a child thereof.\n"
+    "\n"
+    "  This should generally be in the root_out_dir or a subdirectory thereof\n"
+    "  (the root_out_dir will be the same as the root_build_dir for the\n"
+    "  default toolchain, and will be a subdirectory for other toolchains).\n"
+    "  Not putting the output in a subdirectory of root_out_dir can result\n"
+    "  in collisions between different toolchains, so you will need to take\n"
+    "  steps to ensure that your target is only present in one toolchain.\n"
+    "\n"
+    "  Normally the toolchain specifies the output directory for libraries\n"
+    "  and executables (see \"gn help tool\"). You will have to consult that\n"
+    "  for the default location. The default location will be used if\n"
+    "  output_dir is undefined or empty.\n"
+    "\n"
+    "Example\n"
+    "\n"
+    "  shared_library(\"doom_melon\") {\n"
+    "    output_dir = \"$root_out_dir/plugin_libs\"\n"
+    "    ...\n"
+    "  }\n";
+
 const char kOutputName[] = "output_name";
 const char kOutputName_HelpShort[] =
     "output_name: [string] Name for the output file other than the default.";
@@ -1636,6 +1664,7 @@ const VariableInfoMap& GetTargetVariables() {
     INSERT_VARIABLE(Ldflags)
     INSERT_VARIABLE(Libs)
     INSERT_VARIABLE(LibDirs)
+    INSERT_VARIABLE(OutputDir)
     INSERT_VARIABLE(OutputExtension)
     INSERT_VARIABLE(OutputName)
     INSERT_VARIABLE(OutputPrefixOverride)