Re-integrate FEAT_SIGNS code, close #383

- omit FEAT_NETBEANS_INTG and FEAT_SIGN_ICONS
- omit FEAT_GUI blocks
Use the clang 3.4 release tarball instead of apt for clang 3.4.

It appears the llvm.org/apt/ repository isn't always reliable.  So let's
use the release tarball instead.  Also, make using 3.4 conditional, so
we can use the clang 3.3 if things still manage to go awry in the
future.  Note: using 3.3 means that we won't get leak detection.

I left the logic for using llvm.org/apt/, just in case we want try using
it again sometime.