build: introduce ./configure --with-lto

Introduce `--with-lto` configure option to use Link Time Optimization
compiler pass in gcc/clang compilers.

This flag slightly improves performance of C/C++ heavy code:

    tls/throughput.js dur="5" type="buf" size="2": ./out/Release/node-flto: 6.1295 ./out/Release/node: 5.6876 ....... 7.77%
    tls/throughput.js dur="5" type="buf" size="1024": ./out/Release/node-flto: 1567 ./out/Release/node: 1455 ........ 7.70%
    tls/throughput.js dur="5" type="buf" size="1048576": ./out/Release/node-flto: 4167.8 ./out/Release/node: 4026.7 . 3.50%
    tls/throughput.js dur="5" type="asc" size="2": ./out/Release/node-flto: 5.5664 ./out/Release/node: 5.0363 ...... 10.52%
    tls/throughput.js dur="5" type="asc" size="1024": ./out/Release/node-flto: 1462.2 ./out/Release/node: 1336.8 .... 9.38%
    tls/throughput.js dur="5" type="asc" size="1048576": ./out/Release/node-flto: 3947.5 ./out/Release/node: 3847.4 . 2.60%
    tls/throughput.js dur="5" type="utf" size="2": ./out/Release/node-flto: 5.5544 ./out/Release/node: 5.0786 ....... 9.37%
    tls/throughput.js dur="5" type="utf" size="1024": ./out/Release/node-flto: 1328.5 ./out/Release/node: 1255.7 .... 5.80%
    tls/throughput.js dur="5" type="utf" size="1048576": ./out/Release/node-flto: 3051.1 ./out/Release/node: 2985.1 . 2.21%

Fixes: nodejs#7400

common.gypi

@@ -18,6 +18,7 @@
     'node_module_version%': '',
 
     'node_tag%': '',
+    'node_use_lto%': '',
     'uv_library%': 'static_library',
 
     'openssl_fips%': '',
@@ -333,6 +334,13 @@
           [ 'node_shared=="true"', {
             'cflags': [ '-fPIC' ],
           }],
+          ['node_use_lto=="true"', {
+            'conditions': [ [ 'clang==1', {
+              'cflags': [ '-flto' ],
+            }, {
+              'cflags': [ '-flto=16' ],
+            } ] ],
+          }],
         ],
       }],
       ['OS=="android"', {
@@ -391,6 +399,13 @@
               'CLANG_CXX_LIBRARY': 'libc++',
             },
           }],
+          ['node_use_lto=="true"', {
+            'xcode_settings': {
+              'OTHER_CFLAGS': [
+                '-flto',
+              ],
+            },
+          }],
         ],
       }],
       ['OS=="freebsd" and node_use_dtrace=="true"', {

configure

@@ -331,6 +331,11 @@ parser.add_option('--with-lttng',
     dest='with_lttng',
     help='build with Lttng (Only available to Linux)')
 
+parser.add_option('--with-lto',
+    action='store_true',
+    dest='use_lto',
+    help='build with -flto flag (Link Time Optimization)')
+
 parser.add_option('--with-etw',
     action='store_true',
     dest='with_etw',
@@ -838,6 +843,8 @@ def configure_node(o):
   else:
     o['variables']['node_use_lttng'] = 'false'
 
+  o['variables']['node_use_lto'] = b(options.use_lto)
+
   if options.no_ifaddrs:
     o['defines'] += ['SUNOS_NO_IFADDRS']