Provide a way to disable json-c
New in fontconfig 2.13.1, it looks for json-c at configure time and links with it at build time if it was found. This is only used for an optional test in the test suite.
I can't find an obvious way to disable this. It would be good to have an easy way, such as a configure flag, to allow me to specify that I don't want json-c to be looked for or used, even if it is present. (If there is a way to do it and I've missed it, please let me know how.)
This will help when cross-compiling. For example, if I'm trying to build a universal binary of fontconfig, and I have json-c installed non-universally, the build of fontconfig will fail when it can't find the other architecture in the json-c files. See https://trac.macports.org/ticket/57081.