build: enabling lto at configure

This modification allows for compiling with link time optimization (lto)
using the flag --enable-lto.

Refs: #7400

Author: Octavian Soldea

common.gypi

@@ -176,6 +176,17 @@
           ['OS!="mac" and OS!="win"', {
             'cflags': [ '-fno-omit-frame-pointer' ],
           }],
+          ['OS=="linux"', {
+            'variables': {
+              'lto': ' -flto=4 -fuse-linker-plugin -ffat-lto-objects ',
+            },
+            'conditions': [
+              ['enable_lto=="true"', {
+                'cflags': ['<(lto)'],
+                'ldflags': ['<(lto)'],
+              },],
+            ],
+          },],
           ['OS == "android"', {
             'cflags': [ '-fPIE' ],
             'ldflags': [ '-fPIE', '-pie' ]

configure

@@ -157,6 +157,11 @@ parser.add_option("--enable-vtune-profiling",
          "JavaScript code executed in nodejs. This feature is only available "
          "for x32, x86, and x64 architectures.")
 
+parser.add_option("--enable-lto",
+    action="store_true",
+    dest="enable_lto",
+    help="Enable compiling with lto of a binary. This feature is only available "
+         "on linux with gcc and g++.")
 
 parser.add_option("--link-module",
     action="append",
@@ -932,6 +937,23 @@ def configure_node(o):
   else:
     o['variables']['node_enable_v8_vtunejit'] = 'false'
 
+  if flavor != 'linux' and (options.enable_lto):
+    raise Exception(
+      'The lto option is supported only on linux.')
+
+  if flavor == 'linux':
+    if options.enable_lto:
+      version_checked = '5.4.1'
+      for compiler in [(CC, 'c'), (CXX, 'c++')]:
+        ok, is_clang, clang_version, compiler_version = \
+          try_check_compiler(compiler[0], compiler[1])
+        if is_clang or compiler_version < version_checked:
+          raise Exception(
+            'The option --enable-lto is supported for '
+            'gcc and gxx 5.4.1 or newer only.')
+
+  o['variables']['enable_lto'] = b(options.enable_lto)
+
   if flavor in ('solaris', 'mac', 'linux', 'freebsd'):
     use_dtrace = not options.without_dtrace
     # Don't enable by default on linux and freebsd