With unified headers "vprintf(NULL,0)" fails to compile with clang
since the second argument should be a va_list.

diff -u -r ../src-orig/cnf/configure_func.sh ./cnf/configure_func.sh
--- ../src-orig/cnf/configure_func.sh	2017-06-28 11:41:43.391383248 +0200
+++ ./cnf/configure_func.sh	2017-06-28 11:42:10.055083970 +0200
@@ -300,8 +300,8 @@
 checkfunc d_usleep 'usleep' '0' 'unistd.h'
 checkfunc d_ustat 'ustat' '0,NULL' 'sys/types.h unistd.h'
 define d_vfork 'undef' # unnecessary
-checkfunc d_vprintf 'vprintf' 'NULL,0' 'stdio.h'
-checkfunc d_vsnprintf 'vsnprintf' 'NULL,0,NULL,NULL' 'stdio.h'
+define d_vprintf 'define'
+define d_vsnprintf 'define'
 checkfunc d_wait4 'wait4' '0,NULL,0,NULL' 'sys/wait.h'
 checkfunc d_waitpid 'waitpid' '0,NULL,0' 'sys/wait.h'
 checkfunc d_wcrtomb 'wcrtomb' 'NULL,0,NULL' 'wchar.h'