warn when options -a and -T are used together

NEWS

@@ -3,6 +3,8 @@ Noteworthy Changes in ssocr Releases
 
 Version ?.??.? (????-??-??):
 ----------------------------
+ * Print warning when the two options -a, --absolute-threshold and -T,
+   --iter-threshold are used together because -a inhibits -T
  * Improved performance when reading image data via standard input
  * Improved performance when using gray_stretch together with -g
  * Documentation improvements

ssocr.c

@@ -639,6 +639,8 @@ int main(int argc, char **argv)
         exit(99);
     }
   }
+  if((flags & ABSOLUTE_THRESHOLD) && (flags & DO_ITERATIVE_THRESHOLD))
+    fprintf(stderr, "%s: warning: -T has no effect due to -a\n", PROG);
   if(flags & DEBUG_OUTPUT) {
     fprintf(stderr, "================================================================================\n");
     fprintf(stderr, "VERSION=%s\n", VERSION);