[Libguestfs] Add support for musl libc

Hi, I’m trying to compile libguestfs on Alpine Linux, that uses musl libc (lightweight, fast and simple standards-conformance libc). It fails in the check phase due to missing glibc-style extended printf formatters. -- checking for register_printf_specifier... no checking for register_printf_function... no configure: error: in `/tmp/libguestfs/src/libguestfs-1.32.6’: configure: error: No support for glibc-style extended printf formatters. This means you either have a very old glibc (pre-2.0) or you are using some other libc where this is not supported. See `config.log’ for more details -- Could you please add support for musl libc, i.e. avoid using glibc-specific extensions? How often are such non-standard functions used in the libguestfs code-base; can we just patch few occurrences or is it too widespread? Thanks, Jakub Jirutka