build: turn warnings into errors for node sources

This commit attempts to turn compilation warnings into errors for node's
source code.

The motivation for this is to enable these errors to be reported by CI
runs on pull requests and be able to fix them before they land.

Author: danbev

common.gypi

@@ -218,7 +218,12 @@
     # Forcibly disable -Werror.  We support a wide range of compilers, it's
     # simply not feasible to squelch all warnings, never mind that the
     # libraries in deps/ are not under our control.
-    'cflags!': ['-Werror'],
+    'conditions': [
+      ['_target_name!="<(node_lib_target_name)" or '
+        '_target_name!="<(node_core_target_name)"', {
+        'cflags!': ['-Werror'],
+      }],
+    ],
     'msvs_settings': {
       'VCCLCompilerTool': {
         'BufferSecurityCheck': 'true',

node.gyp

@@ -346,6 +346,8 @@
         'node.gypi'
       ],
 
+      'cflags': [ '-Werror', ],
+
       'include_dirs': [
         'src',
         'deps/v8/include'
@@ -528,6 +530,8 @@
         'node.gypi',
       ],
 
+      'cflags': [ '-Werror', ],
+
       'include_dirs': [
         'src',
         '<(SHARED_INTERMEDIATE_DIR)' # for node_natives.h