Ensure that we can propagate the correct library path

On OS X El Capitan we need to explicitly copy LSST_LIBRARY_PATH
to DYLD_LIBRARY_PATH. We use a sconsUtils utility function for this.

hsc/SConscript

@@ -6,11 +6,16 @@ import os
 
 # scons steals our environment away, so we have to reinstate it
 env = lsst.sconsUtils.env.Clone()
-environ = dict((name, os.environ[name]) for name in ("PYTHONPATH", "LD_LIBRARY_PATH",
-                                                     "DYLD_LIBRARY_PATH", "PATH"))
-env.Append(**environ)
-# scons also hacks sys.path, so we need to set PYTHONPATH very explicitly at the start of each command.
-python = "PYTHONPATH=%s " % environ["PYTHONPATH"]
+for name in ("PYTHONPATH", "LD_LIBRARY_PATH",
+             "DYLD_LIBRARY_PATH", "PATH"):
+    if name in os.environ:
+        env.AppendENVPath(name, os.environ[name])
+
+# we may need an explicit library load path specified in the command
+libpathstr = lsst.sconsUtils.utils.libraryLoaderEnvironment()
+
+# We always run these commands with an explicit python rather than relying on the shebang
+python = "{} python".format(libpathstr)
 
 dep = "#python/lsst/obs/subaru/_subaruLib.so" # Dependency required for running the python scripts