5/llvm: Add LLVM as a package for RTEMS.

- Add '%source download <source>' to only download the source
  and do not unpack and prep. This can used when a package
  internally needs another source package.

- Install the staging root only if it is present. A package
  may internally build another package that is not staged as it
  is not suitable for installing.

Updates #3250
Updatew #3797

source-builder/sb/darwin.py

@@ -52,6 +52,7 @@ def load():
         '_var':             ('dir',     'optional', '/usr/local/var'),
         '_prefix':          ('dir',     'optional', '%{_usr}'),
         '__ldconfig':       ('exe',     'none',     ''),
+        '__cmake':          ('exe',     'optional', 'cmake'),
         '__cvs':            ('exe',     'optional', 'cvs'),
         '__xz':             ('exe',     'required', 'xz'),
         'with_zlib':        ('none',    'none',     '--with-zlib=no'),