Undefine symbols before redefining them.

#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