Support configuration of printf options in Meson

[phillipjohnston]

Based on #159, we have a new printf implementation. This uses CMake and has a configuration header input, among a number of options. These options similarly should be exposed to Meson.

One option here is to try to find a simple way to do it in the toplevel libc build. Another option is to create a Meson equivalent. We can either use that exclusively in our fork or work on getting it merged into the eyalroz/printf repository.

[phillipjohnston]

Note this define will be necessary:

// Define this globally (e.g. gcc -DPRINTF_INCLUDE_CONFIG_H ...) to include the
// printf_config.h header file
#ifndef PRINTF_INCLUDE_CONFIG_H
#define PRINTF_INCLUDE_CONFIG_H 0
#endif