json-c: drop host-autoconf dependency

It turns out that the configure script attempts to run autoheader
because the configure.ac timestamp is slightly later than that of
config.h.in. Update the config.h.in timestamp after tarball extract to
avoid autoheader run. With that we can drop the host-autoconf
dependency.

Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>