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
@@ -275,7 +275,7 @@
 checkfunc d_usleep 'usleep'
 checkfunc d_ustat 'ustat'
 define d_vfork 'undef' # unnecessary
-checkfunc d_vprintf 'vprintf' 'NULL,0'
+define d_vprintf 'define'
 checkfunc d_vsnprintf 'vsnprintf'
 checkfunc d_wait4 'wait4'
 checkfunc d_waitpid 'waitpid' '0,NULL,0'