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.

configure.ac

@@ -107,7 +107,6 @@ if ! "${stdpass-false}"; then
 fi
 
 AX_CHECK_BUDDY
-AX_CHECK_GSPNLIB
 
 AC_CHECK_HEADERS([sys/times.h])
 AC_CHECK_FUNCS([times kill alarm])
@@ -173,8 +172,6 @@ AC_CONFIG_FILES([
   doc/org/init.el
   iface/dve2/defs
   iface/dve2/Makefile
-  iface/gspn/defs
-  iface/gspn/Makefile
   iface/Makefile
   lib/Makefile
   src/bin/Makefile