compression: only declare revlog support for available engine

Even if we know that an engine support revlog compression, the Mercurial process
still won't support it if the compression engine is not available.

mercurial/localrepo.py

@@ -643,7 +643,7 b' def gathersupportedrequirements(ui):'
     # Add derived requirements from registered compression engines.
     for name in util.compengines:
         engine = util.compengines[name]
-        if engine.revlogheader():
+        if engine.available() and engine.revlogheader():
             supported.add(b'exp-compression-%s' % name)
 
     return supported