Undefine symbols before redefining them.

lib/curlx.h

@@ -79,7 +79,19 @@
 #ifdef ENABLE_CURLX_PRINTF
 /* If this define is set, we define all "standard" printf() functions to use
    the curlx_* version instead. It makes the source code transparant and
-   easier to understand/patch. */
+   easier to understand/patch. Undefine them first in case _MPRINTF_REPLACE
+   is set. */
+# undef printf
+# undef fprintf
+# undef sprintf
+# undef snprintf
+# undef vprintf
+# undef vfprintf
+# undef vsprintf
+# undef vsnprintf
+# undef aprintf
+# undef vaprintf
+
 # define printf curlx_mprintf
 # define fprintf curlx_mfprintf
 # define sprintf curlx_msprintf