C++ style comments in C89 mode will cause errors only once per input file.

src/cc65/global.c

@@ -49,6 +49,7 @@ unsigned char DebugInfo         = 0;    /* Add debug info to the obj */
 unsigned char PreprocessOnly    = 0;    /* Just preprocess the input */
 unsigned char DebugOptOutput    = 0;    /* Output debug stuff */
 unsigned      RegisterSpace     = 6;    /* Space available for register vars */
+unsigned      AllowNewComments  = 0;    /* Allow new style comments in C89 mode */
 
 /* Stackable options */
 IntStack WritableStrings    = INTSTACK(0);  /* Literal strings are r/w */

src/cc65/global.h

@@ -57,6 +57,7 @@ extern unsigned char    DebugInfo;              /* Add debug info to the obj */
 extern unsigned char    PreprocessOnly;         /* Just preprocess the input */
 extern unsigned char    DebugOptOutput;         /* Output debug stuff */
 extern unsigned         RegisterSpace;          /* Space available for register vars */
+extern unsigned         AllowNewComments;       /* Allow new style comments in C89 mode */
 
 /* Stackable options */
 extern IntStack         WritableStrings;        /* Literal strings are r/w */

src/cc65/preproc.c

@@ -402,8 +402,10 @@ static void NewStyleComment (void)
 /* Remove a new style C comment from line. */
 {
     /* Diagnose if this is unsupported */
-    if (IS_Get (&Standard) < STD_C99) {
+    if (IS_Get (&Standard) < STD_C99 && !AllowNewComments) {
         PPError ("C++ style comments are not allowed in C89");
+        PPNote ("(this will be reported only once per input file)");
+        AllowNewComments = 1;
     }
 
     /* Beware: Because line continuation chars are handled when reading
@@ -1800,6 +1802,9 @@ void PreprocessBegin (void)
 
     /* Remember to update source file location in preprocess-only mode */
     FileChanged = 1;
+
+    /* Enable diagnostics on new style comments in C89 mode */
+    AllowNewComments = 0;
 }