lib/printf.h, libtests/t-printf.c: Bodge to inhibit warning from GCC 9.

lib/printf.h, libtests/t-printf.c: Bodge to inhibit warning from GCC 9.

It seems that GCC 9 now peers closely at field-width specifiers in
`printf' format strings to make sure that they're sensible.  This is a
good thing except when you're trying to test how your `printf'-like
function copes with out-of-range field width specifiers.

Add a new macro `INHIBIT_PRINTF_FORMAT_CHECKING' to turn off the magic
GCC attribute, and use it in `t-printf.c' to prevent GCC from breaking
the build.