* configure.host: Add check for --enable-newlib-io-pos-args
and define WANT_IO_POS_ARGS flag if enabled. Define
the flag by default for x86-linux configurations.
* configure.in: Add support for --enable-newlib-io-pos-args.
* libc/configure.in: Ditto.
* configure: Regenerated.
* libc/configure: Ditto.
* libc/stdio/Makefile.am: Specify -fshort-enums for compiling
vfprintf.c and vfiprintf.c.
* libc/stdio/Makefile.in: Regenerated.
* libc/stdio/vfprintf.c: Add positional argument support that
is enabled by compiling with -DWANT_IO_POS_ARGS.