added Scintilla submodule

This links to the `sciteco-dev` branch at git@github.com:rhaberkorn/scintille-mirror.git,
which is actually Scintilla 3.4.4.
automatically build Scintilla as part of SciTECO's build system:
this should simplify building SciTECO for new users

 * compiler and archiver are passed down from Autoconf,
   so cross-compiling should work transparently
 * `make clean` will also clean the Scintilla source tree
 * there is no longer any need for "source bundles" as
   tar balls also contain Scintilla/Scinterm now
 * building from Git is not much more difficult than building
   from a tar ball
 * The versions of Scintilla/Scinterm embedded as submodules
   already contain all the patches necessary (currently none are
   necessary), so there's no need to have patch files in the
   repository
 * INSTALL instructions have been rewritten
 * the --with-scintilla and --with-scinterm site-config options
   have been kept. But they should be rarely necessary now.