OS X: Make the configure script work with either Qt4 or Qt5.

 * The default QMAKESPEC is unsuitable for both Qt4 and Qt5, but Qt4 and
   Qt5 have different spec files for OS X.

 * Add a --with-clang option for specifying the path to qmake.
OS X: Stop using an app bundle for sourceweb (temporarily?)

 * The current workflow for using the sourceweb GUI is to enter a directory
   containing an index file, from the command-line, then run sourceweb.
   This is inconvenient when the binary is at, say,
   /usr/local/sourceweb.app/Contents/MacOS/sourceweb.

 * Installing binaries/libraries to /usr/local/{bin,libexec} makes sense
   to me, but I'm not sure about /usr/local/sourceweb.app.

 * IIRC, opening a bundle from the command-line started the process with a
   different cwd than the one in the terminal.  This currently breaks
   (assertion failure when the index can't be loaded).

 * Perhaps the workflow in sourceweb should change -- maybe one process
   could open multiple index files?  Or, the process could be running with
   no index files open?  Or, the process could prompt the user to enter a
   path to an index file on startup?
Add build instructions for OS X to the README.