scons: Add warning for missing declarations

This patch enables warnings for missing declarations. To avoid issues
with SWIG-generated code, the warning is only applied to non-SWIG
code.

src/SConscript

@@ -899,9 +899,17 @@ def makeEnv(label, objsfx, strip = False, **kwargs):
                                  '-Wno-unused-value'])
         if compareVersions(env['GCC_VERSION'], '4.6') >= 0:
             swig_env.Append(CCFLAGS='-Wno-unused-but-set-variable')
+
+        # Add additional warnings here that should not be applied to
+        # the SWIG generated code
+        new_env.Append(CXXFLAGS='-Wmissing-declarations')
     if env['CLANG']:
         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')
+
     werror_env = new_env.Clone()
     werror_env.Append(CCFLAGS='-Werror')