c++ complains about __VA_ARGS__

The following code has been compiled with gcc-5.4.0 with no issues:

% gcc -W -Wall a.c
...

#include <stdio.h>
#include <stdarg.h>

static int debug_flag;
static void debug(const char *fmt, ...)
{
   va_list ap;

   va_start(ap, fmt);
   vfprintf(stderr, fmt, ap);
   va_end(ap);
}

#define DEBUG(...)               \
   do {                 \
      if (debug_flag) {       \
         debug("DEBUG:"__VA_ARGS__);  \
      }              \
   } while(0)

int main(void)
{
   int dummy = 10;
   debug_flag = 1;
   DEBUG("debug msg dummy=%d\n", dummy);
   return 0;
}

However compiling this with g++ has interesting effects:

% g++ -W -Wall -std=c++11 a.c
a.c: In function ‘int main()’:
a.c:18:10: error: unable to find string literal operator ‘operator""__VA_ARGS__’ with ‘const char [8]’, ‘long unsigned int’ arguments
    debug("DEBUG: "__VA_ARGS__); \

% g++ -W -Wall -std=c++0x
<same error>

% g++ -W -Wall -std=c++03
<no errors>

Changing debug("DEBUG:"__VA_ARGS__); to debug("DEBUG:" __VA_ARGS__); i.e. space before __VA_ARGS__ enables to compile with all three -std= options.

What is the reason for such behaviour? Thanks.