warn about declarations after statements when compiling with gcc.

configure

@@ -387,6 +387,11 @@ else
 fi
 
 # a final configuration tuning
+W_OPTIONS="declaration-after-statement"
+for i in $W_OPTIONS; do
+    O_PRESENT="$($cc -v --help 2>&1 | grep -- -W$i)"
+    if test -n "$O_PRESENT"; then  CFLAGS="$CFLAGS -W$i"; fi
+done
 W_OPTIONS="deprecated-declarations strict-aliasing pointer-sign sign-compare unused-result uninitialized"
 for i in $W_OPTIONS; do
     O_PRESENT="$($cc -v --help 2>&1 | grep -- -W$i)"