How can I include expansions of the __FILE__ and __LINE__ macros in a general-purpose debugging macro?

One solution involves writing your debug macro in terms of a varargs function , and an auxiliary function which stashes the values of __FILE__ and __LINE__ away in static variables, as in:
#include <stdio.h>
#include <stdarg.h>

void debug(const char *, ...);
void dbginfo(int, const char *);
#define DEBUG dbginfo(__LINE__, __FILE__), debug

static const char *dbgfile;
static int dbgline;

void dbginfo(int line, const char *file)
{
dbgfile = file;
dbgline = line;
}

void debug(const char *fmt, ...)
{
va_list argp;
fprintf(stderr, "DEBUG: "%s", line %d: ", dbgfile, dbgline);
va_start(argp, fmt);
vfprintf(stderr, fmt, argp);
va_end(argp);
fprintf(stderr, "\n");
}

With this machinery in place, a call to
DEBUG("i is %d", i);
expands to
dbginfo(__LINE__, __FILE__), debug("i is %d", i);
and prints something like
DEBUG: "x.c", line 10: i is 42
A cunning improvement is the idea of having the stashing function return a pointer to the bona-fide varargs function:
void debug(const char *, ...);
void (*dbginfo(int, const char *))(const char *, ...);
#define DEBUG (*dbginfo(__LINE__, __FILE__))

void (*dbginfo(int line, const char *file))(const char *, ...)
{
dbgfile = file;
dbgline = line;
return debug;
}

With these definitions,
DEBUG("i is %d", i);
gets expanded to
(*dbginfo(__LINE__, __FILE__))("i is %d", i);
Another, perhaps easier way might simply be to
#define DEBUG printf("DEBUG: "%s", line %d: ", \
__FILE__,__LINE__),printf

Now,
DEBUG("i is %d", i);
simply expands to
printf("DEBUG: "%s", line %d: ",
__FILE__,__LINE__),printf("i is %d", i);
Finally, you may be able to use the
#define _ ,