Add Cmakefile and fix compiler warning.

[randy-armstrong]

A cmake file is handy for devs who target multiple OSes including windows.
It has not been tested on non-Windows platforms but it should work.

[hawicz]

please split the json_object.c change off separately. (why is that needed?)

[randy-armstrong]

reverted the changes to json_object.c

It is necessary because code should not have warnings.

[hawicz]

I included your change as a single commit c5b5a98.

If you think there's something wrong with the json_object_object_add_ex() function please open a separate ticket and include the actual warning that you're seeing, as it compiles just fine for me.
Also, even if you are seeing some kind of warning, dropping the const qualifiers is almost certainly the wrong fix.