Don't hash -U in the preprocessor mode

NEWS.txt

@@ -100,6 +100,10 @@ New features and improvements
     - For debugging purposes, the variable `CCACHE_VERBOSE` may now be set to
       make ccache print executed commands to standard output.
 
+    - The following options are no longer hashed in the preprocessor mode:
+      `-imacros`, `-iprefix`, `-iwithprefix`, `-iwithprefixbefore`,
+      `-nostdinc`, `-nostdinc++` and `-U`.
+
 
 Bug fixes
 ~~~~~~~~~

ccache.c

@@ -863,7 +863,8 @@ static struct file_hash *calculate_object_hash(
 				    strcmp(args->argv[i], "-idirafter") == 0 ||
 				    strcmp(args->argv[i], "-isystem") == 0 ||
 				    strcmp(args->argv[i], "-nostdinc") == 0 ||
-				    strcmp(args->argv[i], "-nostdinc++") == 0) {
+				    strcmp(args->argv[i], "-nostdinc++") == 0 ||
+				    strcmp(args->argv[i], "-U") == 0) {
 					/* Skip from hash. */
 					i++;
 					continue;