Ignore NameError in unused conditional configuration sections

Adding new names for expression is currently not possible because
buildout aborts before it tries to upgrade (in-place or bootstrap).

src/zc/buildout/buildout.py

@@ -1338,7 +1338,9 @@ class Buildout(DictMixin):
                 data = self._raw[section]
             except KeyError:
                 raise MissingSection(section)
-
+            e = data.get('__unsupported_conditional_expression__')
+            if e:
+                raise e
             options = self.Options(self, section, data)
             self._data[section] = options
             options._initialize()

src/zc/buildout/configparser.py

@@ -187,7 +187,12 @@ def parse(fp, fpname, exp_globals=dict):
                     if not context:
                         context = exp_globals()
                     # evaluated expression is in list: get first element
-                    section_condition = eval(expr, context)[0]
+                    try:
+                        section_condition = eval(expr, context)[0]
+                    except NameError as x:
+                        sections.setdefault(sectname, {})[
+                            '__unsupported_conditional_expression__'] = x
+                        continue
                     # finally, ignore section when an expression
                     # evaluates to false
                     if not section_condition:
@@ -239,6 +244,8 @@ def parse(fp, fpname, exp_globals=dict):
         section = sections[sectname]
         for name in section:
             value = section[name]
+            if isinstance(value, NameError):
+                continue
             if value[:1].isspace():
                 section[name] = leading_blank_lines.sub(
                     '', textwrap.dedent(value.rstrip()))