Add node_enable_deprecated_declarations_warnings GN flag (nodejs#181)

* Add node_enable_deprecated_declarations_warnings GN flag

Warnings about using deprecated declarations were disabled by default
which made it hard to ensure that Node doesn't use V8's deprecated
Apis.

The flag allows enabling deprecated warnings and suppresses (hopefully)
known issues with using deprecated functionality in c-api.

The flag is off by default which preserves the existing behavior.

Drive-by: fix deps/openssl/unofficial.gni by exposing the required
OpenSSL compatibility level (OPENSSL_API_COMPAT) via public_configs.

deps/openssl/unofficial.gni

@@ -16,6 +16,7 @@ template("openssl_gn_build") {
       "openssl/crypto/include",
       "openssl/include",
     ]
+    defines = [ "OPENSSL_API_COMPAT=0x10100001L" ]
   }
 
   config("openssl_internal_config") {
@@ -26,7 +27,6 @@ template("openssl_gn_build") {
 
     defines = [
       "MODULESDIR=\"deps/openssl/lib/openssl-modules\"",
-      "OPENSSL_API_COMPAT=0x10100001L",
       "STATIC_LEGACY",
     ] + gypi_values.openssl_default_defines_all
     if (is_win) {

unofficial.gni

@@ -6,6 +6,12 @@ import("node.gni")
 import("$node_v8_path/gni/snapshot_toolchain.gni")
 import("$node_v8_path/gni/v8.gni")
 
+declare_args() {
+  # Enable warnings about usages of deprecated declarations and suppress
+  # known issues with c-ares.
+  node_enable_deprecated_declarations_warnings = false
+}
+
 # The actual configurations are put inside a template in unofficial.gni to
 # prevent accidental edits from contributors.
 template("node_gn_build") {
@@ -33,7 +39,7 @@ template("node_gn_build") {
       defines += [ "HAVE_INSPECTOR=0" ]
     }
     if (node_use_node_code_cache) {
-      defines += [ "NODE_USE_NODE_CODE_CACHE=1"]
+      defines += [ "NODE_USE_NODE_CODE_CACHE=1" ]
     }
     if (v8_enable_i18n_support) {
       defines += [ "NODE_HAVE_I18N_SUPPORT=1" ]
@@ -64,7 +70,6 @@ template("node_gn_build") {
     libs = []
     cflags = [ "-Wno-microsoft-include" ]
     cflags_cc = [
-      "-Wno-deprecated-declarations",
       "-Wno-extra-semi",
       "-Wno-implicit-fallthrough",
       "-Wno-macro-redefined",
@@ -82,6 +87,10 @@ template("node_gn_build") {
       "-Wno-unused-function",
     ]
 
+    if (!node_enable_deprecated_declarations_warnings) {
+      cflags_cc += [ "-Wno-deprecated-declarations" ]
+    }
+
     if (current_cpu == "x86") {
       node_arch = "ia32"
     } else {
@@ -100,6 +109,11 @@ template("node_gn_build") {
       "NODE_REPORT"
     ]
 
+    if (node_enable_deprecated_declarations_warnings) {
+      # Refs: https://github.com/nodejs/node/issues/52464
+      defines += [ "CARES_NO_DEPRECATED=1" ]
+    }
+
     if (is_win) {
       defines += [
         "NOMINMAX",