Move check for xulrunner to external file in environ

This change is needed to support packaging localizations in FieldWorks
(FWNX-489).  Building and packaging localizations does not need xulrunner,
and can be messed up by looking for it and not finding it.

This commit was cherry picked from 61f7435 on support/7.0-Linux

Change-Id: I367786fd73a6aba4438839aa4954b16a6a60a75f