Scaffolding for defining settings.

Settings can be defined globally or per-ISA. They are available to code through
a generated Settings struct with accessor methods per setting.

meta/cretonne/settings.py

@@ -0,0 +1,13 @@
+"""
+Cretonne shared settings.
+
+This module defines settings are are relevant for all code generators.
+"""
+
+from . import SettingGroup, BoolSetting
+
+group = SettingGroup('shared')
+
+enable_simd = BoolSetting("Enable the use of SIMD instructions", default=True)
+
+group.close(globals())