[fix/opti] Conditional configuration sections: compute values at most once, add 'multiarch'

For example, existing values were not enough to distinguish
'arm-linux-gnueabi' from 'arm-linux-gnueabihf'.

The 'multiarch' value is the output of

  $CC -dumpmachine

where CC defaults to 'gcc'.

See also https://wiki.debian.org/Multiarch/Tuples

src/zc/buildout/buildout.py

@@ -2051,6 +2051,49 @@ def _default_globals():
 
     return globals_defs
 
+
+class _default_globals(dict):
+    """
+    Make sure parser context is computed at most once,
+    even if several files are parsed.
+    And compute some values only if accessed.
+
+    If __getitem__ raises, _doing() calls .get('__doing__'),
+    but that's not the only reason to subclass dict:
+    CPython requests it (for performance reasons?). PyPy does not care.
+    """
+    # XXX: The following line is only to keep access to the overridden global.
+    #      If pushed upstream, proper naming would avoid such hack.
+    #      Meanwhile, the patch consists only in this drop-in class
+    #      and that's easier to maintain.
+    _default_globals = staticmethod(_default_globals)
+
+    def __getitem__(self, key):
+        cls = self.__class__
+        try:
+            context = self.context
+        except AttributeError:
+            context = self.context = cls._default_globals()
+        try:
+            return context[key]
+        except KeyError as e:
+            try:
+                value = getattr(self, key)
+            except AttributeError:
+                pass
+            else:
+                value = context[key] = value()
+                return value
+            raise e # BBB: On Python 3, a bare 'raise' is enough.
+
+    def multiarch(self):
+        args = os.getenv('CC') or 'gcc', '-dumpmachine'
+        self['__doing__'] = '%r', args
+        m = subprocess.check_output(args, universal_newlines=True).rstrip()
+        del self['__doing__']
+        return m
+
+
 variable_template_split = re.compile('([$]{[^}]*})').split