utf8.h: Change name of non-C-conforming macro

It shouldn't have a leading underscore; change to trailing

Author: khwilliamson

utf8.c

@@ -1824,7 +1824,7 @@ Perl__utf8n_to_uvchr_msgs_helper(const U8 *s,
                         /* If we don't know for sure that the input length is
                          * valid, avoid as much as possible reading past the
                          * end of the buffer */
-                        int printlen = (flags & _UTF8_NO_CONFIDENCE_IN_CURLEN)
+                        int printlen = (flags & UTF8_NO_CONFIDENCE_IN_CURLEN_)
                                        ? (int) (s - s0)
                                        : (int) (send - s0);
                         pack_warn = packWARN(WARN_UTF8);

utf8.h

@@ -1191,7 +1191,7 @@ point's representation.
 #define UTF8_WARN_FE_FF                 UTF8_WARN_PERL_EXTENDED
 
 #define UTF8_CHECK_ONLY			0x8000
-#define _UTF8_NO_CONFIDENCE_IN_CURLEN   0x10000  /* Internal core use only */
+#define UTF8_NO_CONFIDENCE_IN_CURLEN_   0x10000  /* Internal core use only */
 
 /* For backwards source compatibility.  They do nothing, as the default now
  * includes what they used to mean.  The first one's meaning was to allow the