Fix bug 2193: http://scons.tigris.org/issues/show_bug.cgi?id=2193

Fix bug 2193: scons.tigris.org/issues/show_bug.cgi?id=2193

Ap/PrependENVPath now accept paths starting with # and Dirs.  Added
optional _canonicalize arg to SCons.Util.Ap/PrependPath, and pass an
implementation of that into them from Environment.Ap/PrependENVPath.

Can't just always do the canonicalization in SCons.Util because there
is no env there to get the fs to convert a #-prefixed path to the
proper path.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@3867 fdb21ef1-2011-0410-befe-b5e4ea1792b1