gspn: remove the interface with GreatSPN

It hasn't been tested for several year, may not even compile, has
to be linked with source code that isn't even publicly available,
and its presence was the only reason to keep some inefficient
code in gtec.cc and friends.

* iface/gspn/: Delete this directory.
* iface/Makefile.am, configure.ac, README: Adjust.
* m4/gspnlib.m4: Delete.
* src/sanity/Makefile.am: Do not use LIBGSPN_CPPFLAGS.

README

@@ -61,16 +61,6 @@ should read the file INSTALL for generic instructions.
 In addition to its usual options, ./configure will accept some
 flags specific to Spot:
 
-  --with-gspn=DIR
-    Turns on GreatSPN support.  DIR should designate the root of
-    GreatSPN source tree.  (./configure will then run
-    DIR/SOURCES/contrib/version.sh to find the GreatSPN build tree.)
-
-    GreatSPN had to be modified in order to be used as a library
-    (thanks Soheib Baarir and Yann Thierry-Mieg for this work), and
-    presently these modifications are only available on the GreatSPN
-    CVS repository hosted by the Università di Torino.
-
   --with-included-buddy
     After you have installed Spot the first time, a modified version
     of BuDDy will be installed.  The next time you reconfigure Spot,
@@ -201,8 +191,6 @@ wrap/             Wrappers for other languages.
       ajax/       LTL-to-TGBA translator with web interface, using Ajax.
 iface/            Interfaces to other libraries.
    dve2/          Interface with DiVinE2.
-   gspn/          GreatSPN interface.
-      examples/   Supporting models used by the test cases.
 
 Third party software
 --------------------