[clang] Set correct clang warning flags.

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 8ec06fd5e011d44ecd7368f138b99f50fe7582b3..8d3a059ed77c151ef41683488d8d54297a11b0f6 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -199,8 +199,21 @@
         ],
         'conditions': [
           ['os_posix == 1 and OS != "mac"', {
+            # We don't want to get warnings from third-party code,
+            # so remove any existing warning-enabling flags like -Wall.
             'cflags!': [
+              '-pedantic',
+              '-Wall',
               '-Werror',
+              '-Wextra',
+            ],
+            # Clang considers the `register` keyword as deprecated, but
+            # icu uses it all over the place.
+            'cflags+': [
+              '-Wno-deprecated-register',
+            ],
+            'cflags_cc!': [
+              '-Wnon-virtual-dtor',
             ],
           }],
           ['OS == "mac"', {
@@ -286,7 +299,6 @@
         'cflags': [
           '-Wall',
           '<(werror)',
-          '-W',
           '-Wno-unused-parameter',
           '-Wno-long-long',
           '-pthread',
@@ -317,7 +329,6 @@
         'cflags': [
           '-Wall',
           '<(werror)',
-          '-W',
           '-Wno-unused-parameter',
           '-fno-exceptions',
           # Don't warn about the "struct foo f = {0};" initialization pattern.
@@ -460,7 +471,6 @@
           'WARNING_CFLAGS': [
             '-Wall',
             '-Wendif-labels',
-            '-W',
             '-Wno-unused-parameter',
             # Don't warn about the "struct foo f = {0};" initialization pattern.
             '-Wno-missing-field-initializers',