scons: Enable -Wextra by default

Make best use of the compiler, and enable -Wextra as well as
-Wall. There are a few issues that had to be resolved, but they are
all trivial.

src/SConscript

@@ -1018,11 +1018,6 @@ def makeEnv(env, label, objsfx, strip = False, **kwargs):
     # the SWIG generated code
     swig_env.Append(CCFLAGS=['-Wno-unused-label', '-Wno-unused-value'])
 
-    # Add additional warnings here that should not be applied to
-    # the SWIG generated code
-    new_env.Append(CXXFLAGS=['-Wmissing-declarations',
-                             '-Wdelete-non-virtual-dtor'])
-
     if env['GCC']:
         # Depending on the SWIG version, we also need to supress
         # warnings about uninitialized variables and missing field
@@ -1030,7 +1025,8 @@ def makeEnv(env, label, objsfx, strip = False, **kwargs):
         swig_env.Append(CCFLAGS=['-Wno-uninitialized',
                                  '-Wno-missing-field-initializers',
                                  '-Wno-unused-but-set-variable',
-                                 '-Wno-maybe-uninitialized'])
+                                 '-Wno-maybe-uninitialized',
+                                 '-Wno-type-limits'])
 
         # Only gcc >= 4.9 supports UBSan, so check both the version
         # and the command-line option before adding the compiler and
@@ -1041,13 +1037,9 @@ def makeEnv(env, label, objsfx, strip = False, **kwargs):
             new_env.Append(LINKFLAGS='-fsanitize=undefined')
 
     if env['CLANG']:
-        swig_env.Append(CCFLAGS=[
-                # Some versions of SWIG can return uninitialized values
-                '-Wno-sometimes-uninitialized',
-                # Register storage is requested in a lot of places in
-                # SWIG-generated code.
-                '-Wno-deprecated-register',
-                ])
+        swig_env.Append(CCFLAGS=['-Wno-sometimes-uninitialized',
+                                 '-Wno-deprecated-register',
+                                 '-Wno-tautological-compare'])
 
         # All supported clang versions have support for UBSan, so if
         # asked to use it, append the compiler and linker flags.