* HACKING, src/sanity/style.test: NULL is not portable, prohibit it.

* iface/gspn/ssp.cc, src/ltltest/reduc.cc, src/ltltest/syntimpl.cc,
src/ltlvisit/basereduc.cc, src/ltlvisit/reducform.cc,
src/ltlvisit/syntimpl.cc: Use 0 instead of NULL.

src/sanity/style.test

@@ -113,6 +113,9 @@ for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
     grep -e 'return[	 ]*[(][^(]*[)];' $tmp &&
       diag 'No useless parentheses after delete.'
 
+    grep  'NULL' $tmp &&
+      diag 'Use 0 instead of NULL.  NULL is not portable.'
+
     $fail && echo "$file" >>failures
   done
 done