json-c: bump to version 0.13

Use upstream provided tarball; drop autoreconf.

Drop -Werror removal patch; json-c builds fine with gcc 7 without this
patch.

Drop upstream libm patch.

Drop --disable-oldname-compat; removed upstream.

Add license file hash.

Enable parallel build. The commit introducing MAKE1, ecd0f65a
(json-c: bump to version 0.11) does not explain the reason for that. A
sequence of a few builds on a multi-core system never faild.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>