Update the README for the new approach to supporting Linux.

 * The code is tested with Qt 4.6, which is still in use in various
   currently-supported distributions.

 * Use the clang-redist-linux packages.  Remove the instructions for
   building Clang from source -- they should be unnecessary now.  If they
   become necessary later, they should be reintroduced elsewhere, perhaps
   using a GitHub Wiki page.