Add support for enumerated settings.

The EnumSetting objects can take one of 256 named values.

meta/cretonne/settings.py

@@ -4,10 +4,22 @@ Cretonne shared settings.
 This module defines settings are are relevant for all code generators.
 """
 
-from . import SettingGroup, BoolSetting
+from . import SettingGroup, BoolSetting, EnumSetting
 
 group = SettingGroup('shared')
 
-enable_simd = BoolSetting("Enable the use of SIMD instructions", default=True)
+opt_level = EnumSetting(
+        """
+        Optimization level:
+
+        - default: Very profitable optimizations enabled, none slow.
+        - best: Enable all optimizations
+        - fastest: Optimize for compile time by disabling most optimizations.
+        """,
+        'default', 'best', 'fastest')
+
+enable_simd = BoolSetting(
+        """Enable the use of SIMD instructions.""",
+        default=True)
 
 group.close(globals())