AC_CHECK_MEMBER() fails if member is not castable to boolean

[H. Peter Anvin]

From one of my own programs, trying to look for in_pktinfo.ipi_addr, 
which is a struct in_addr:

----------------------------------

configure:8582: checking for struct in_pktinfo.ipi_addr
configure:8615: gcc -c -g -O2 -D_XPG4_2 -D_XOPEN_SOURCE -D__EXTENSIONS__ 
-D_BSD_SOURCE -D_ISO9X_SOURCE -D_OSF_SOU
RCE -D_XOPEN_SOURCE_EXTENDED -W -Wall -Wpointer-arith 
-Wbad-function-cast -Wstrict-prototypes -Wmissing-prototype
s -Wmissing-declarations -Wnested-externs -Winline -Wwrite-strings 
-Wundef -Wshadow -Wsign-compare -pipe  conftes
t.c >&5
configure:8605: warning: function declaration isn't a prototype
configure: In function `main':
configure:8607: invalid operands to binary !=
configure:8618: $? = 1
configure: failed program was:
#line 8588 "configure"
#include "confdefs.h"

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <sys/uio.h>

#ifdef F77_DUMMY_MAIN
#  ifdef __cplusplus
     extern "C"
#  endif
   int F77_DUMMY_MAIN() { return 1; }
#endif
int
main ()
{
static struct in_pktinfo ac_aggr;
if (ac_aggr.ipi_addr)
return 0;
  ;
  return 0;
}
configure:8634: result: no

----------------------------------------

The problem here, of course, is that ac_aggr.ipi_addr isn't castable to 
int (for old C compilers) or _Bool (for newer C compilers), so
"if (ac_aggr.ipi_addr)" is bogus.

A better variation might be the following:

if ( &(ac_aggr.ipi_addr) != (void *)0 )
  return 0;

This, however, will fail if ipi_addr is a bitfield type.  It might be 
useful to have a different macro for bitfield types, I don't know.

        -hpa