build: add --error-on-warn configure flag

This commit adds a configuration time flag named error-on-warn:
$ ./configure --help | grep -A1 error-on-warn
  --error-on-warn       Turn compiler warnings into errors for node core
                        sources.

The motivation for this is that CI jobs can use this flag to turn
warnings into errors.

common.gypi

@@ -26,6 +26,7 @@
     'uv_library%': 'static_library',
 
     'clang%': 0,
+    'error_on_warn%': '0',
 
     'openssl_fips%': '',
     'openssl_no_asm%': 0,
@@ -219,8 +220,9 @@
     # simply not feasible to squelch all warnings, never mind that the
     # libraries in deps/ are not under our control.
     'conditions': [
-      ['_target_name!="<(node_lib_target_name)" or '
-        '_target_name!="<(node_core_target_name)"', {
+      [ 'error_on_warn=="true" and '
+        '(_target_name!="<(node_lib_target_name)" or '
+          '_target_name!="<(node_core_target_name)")', {
         'cflags!': ['-Werror'],
       }],
     ],

configure.py

@@ -117,6 +117,11 @@
     choices=valid_os,
     help='operating system to build for ({0})'.format(', '.join(valid_os)))
 
+parser.add_option('--error-on-warn',
+    action='store_true',
+    dest='error_on_warn',
+    help='Turn compiler warnings into errors for node core sources.')
+
 parser.add_option('--gdb',
     action='store_true',
     dest='gdb',
@@ -1018,6 +1023,7 @@ def configure_node(o):
   o['variables']['node_install_npm'] = b(not options.without_npm)
   o['variables']['debug_node'] = b(options.debug_node)
   o['default_configuration'] = 'Debug' if options.debug else 'Release'
+  o['variables']['error_on_warn'] = b(options.error_on_warn)
 
   host_arch = host_arch_win() if os.name == 'nt' else host_arch_cc()
   target_arch = options.dest_cpu or host_arch

node.gyp

@@ -346,8 +346,6 @@
         'node.gypi'
       ],
 
-      'cflags': [ '-Werror', ],
-
       'include_dirs': [
         'src',
         'deps/v8/include'
@@ -377,6 +375,9 @@
       'msvs_disabled_warnings!': [4244],
 
       'conditions': [
+        [ 'error_on_warn=="true"', {
+          'cflags': ['-Werror'],
+        }],
         [ 'node_intermediate_lib_type=="static_library" and '
             'node_shared=="true" and OS=="aix"', {
           # For AIX, shared lib is linked by static lib and .exp. In the
@@ -530,8 +531,6 @@
         'node.gypi',
       ],
 
-      'cflags': [ '-Werror', ],
-
       'include_dirs': [
         'src',
         '<(SHARED_INTERMEDIATE_DIR)' # for node_natives.h
@@ -753,6 +752,9 @@
       'msvs_disabled_warnings!': [4244],
 
       'conditions': [
+        [ 'error_on_warn=="true"', {
+          'cflags': ['-Werror'],
+        }],
         [ 'node_builtin_modules_path!=""', {
           'defines': [ 'NODE_BUILTIN_MODULES_PATH="<(node_builtin_modules_path)"' ]
         }],