Increase warning level for _decimal.so and libmpdec build when using gcc.

setup.py

@@ -1995,6 +1995,11 @@ class PyBuildExt(build_ext):
         if not sysconfig.get_config_var('WITH_THREAD'):
             define_macros.append(('WITHOUT_THREADS', 1))
 
+        # Increase warning level for gcc:
+        if 'gcc' in cc:
+            extra_compile_args.extend(['-Wextra',
+                                       '-Wno-missing-field-initializers'])
+
         # Uncomment for extra functionality:
         #define_macros.append(('EXTRA_FUNCTIONALITY', 1))
         ext = Extension (