Avoid calling the dbpoll function if it is not declared in the
<sybdb.h> header.  The freetds version does not declare it by
default.  This prevents build failures with future compilers that
do not support implicit function declarations.

diff --git a/backends/mssql/mssql_basic.c b/backends/mssql/mssql_basic.c
index 0506df8f4af3b90c..904a7411a0e3dc80 100644
--- a/backends/mssql/mssql_basic.c
+++ b/backends/mssql/mssql_basic.c
@@ -321,7 +321,9 @@ static int mssql_odbx_result( odbx_t* handle, odbx_result_t** result, struct tim
 		DBPROCESS* cdbproc;
 
 		if( timeout != NULL ) { ms = timeout->tv_sec * 1000 + timeout->tv_usec / 1000; }
+#ifdef HAVE_SYBDB_DBPOLL
 		if( dbpoll( dbproc, ms, &cdbproc, &reason ) == FAIL ) { return -ODBX_ERR_BACKEND; }
+#endif
 		if( reason != DBRESULT ) { return ODBX_RES_TIMEOUT; }   // timeout
 
 		caux->firstresult = 0;
diff --git a/configure.ac b/configure.ac
index 2f74bf8a649c7f4d..d568a3bd53114a40 100644
--- a/configure.ac
+++ b/configure.ac
@@ -168,6 +168,9 @@ int main() { isc_attach_database( NULL, 0, "", NULL, 0, NULL ); return 0; }
 				[AC_MSG_ERROR([mssql library (libsybdb) not found])]
 			)
 			AC_CHECK_MEMBER([struct dbdaterec.year], [AC_DEFINE([HAVE_LIBSYBDB_MSLIB], 1, [Have mslib type of "struct dbdaterec"])], [], [[#include <sybdb.h>]])
+			AC_TRY_COMPILE([#include <sybdb.h>],
+			[RETCODE (*dbpoll_ptr) (void) = ((RETCODE (*) (void)) dbpoll)],
+			[AC_DEFINE([HAVE_SYBDB_DBPOLL], 1, [dbpoll declared in <sybdb.h>])])
 			odbx_saved_libs=${LIBS}; LIBS=""
 			AC_SEARCH_LIBS([dgettext], [intl], [LIBMSSQL="$LIBMSSQL $LIBS"])
 			LIBS=${odbx_saved_libs}