darwin: Make the xz executable path based.

The xz tool is not provided in Xcode command line tools and needs to
built or obtained somehow. This path can be any where so relax the
need for an absolute path.

Close #3280

source-builder/sb/darwin.py

@@ -53,7 +53,7 @@ def load():
         '_prefix':          ('dir',     'optional', '%{_usr}'),
         '__ldconfig':       ('exe',     'none',     ''),
         '__cvs':            ('exe',     'optional', 'cvs'),
-        '__xz':             ('exe',     'required', '%{_usr}/bin/xz'),
+        '__xz':             ('exe',     'required', 'xz'),
         'with_zlib':        ('none',    'none',     '--with-zlib=no'),
         '_forced_static':   ('none',    'none',     ''),
         '_ld_library_path': ('none',    'none',     'DYLD_LIBRARY_PATH')