Bug 1098135 - Traverse directories with misc rules; r=glandium

When the misc tier was added, only directories with misc-associated
variables from moz.build were traversed. This patch adds a dummy
variable to moz.build whose presence will add the directory to the misc
tier.

This will enable us to aggressively convert existing libs:: rules
to the misc tier.

--HG--
extra : rebase_source : c5477010c7d87fafa512f4fbbf1d2b995e07ffbd
extra : amend_source : a54eee6254ef2d63ef6b247d45e38eda2ab88fef
extra : histedit_source : c0f3352e84ec18ca7b88571cd5f9c2a33ab8eda9
Bug 1098135 - Convert some rules to misc tier; r=glandium

--HG--
extra : rebase_source : 1a486bf0ff2e5b3be637f3725cd04b6073e074b8