hghave: add demandimport checking

tests/hghave.py

@@ -530,6 +530,10 b' def has_debhelper():'
                          br'other supported Python versions')
     return dpkg and dh and dh_py2
 
+@check("demandimport", "demandimport enabled")
+def has_demandimport():
+    return os.environ.get('HGDEMANDIMPORT') != 'disable'
+
 @check("absimport", "absolute_import in __future__")
 def has_absimport():
     import __future__