This is the root cause of bug #1143025 and bug #1080227, which cause Minetest to crash when using system jsoncpp. The patch referenced below is already in the bundled version (so, it is a minor fork, basically.)
This was traced down in Debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=753560), and the patch is included https://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;filename=patch;att=1;bug=753560
It seems logically correct.
As noted in the Debian bug, updating to the latest github jsoncpp does fix the problem, but it is an ABI break with no soname change -- everything that uses jsoncpp will need to be changed to Require the newer version and rebuilt. That's kind of a mess. (Possibly should be done in Rawhide one of these days with some coordination of dependent-package owners.)
In the meantime, possibly consider adding this patch to F20 and F21 (and possibly rawhide). (Unfortunately, it makes `scons check` fail, as the patch does not come with a test.)
In case it helps, I built a jsoncpp 0.7.0 snapshot RPM at http://mattdm.org/misc/fedora/jsoncpp-0.7.0-0.1.20140917gitdf26dae.src.rpm and a Copr at http://copr.fedoraproject.org/coprs/mattdm/minetest/
I will patch for F21 and F20, and update for rawhide as you suggested.
jsoncpp-0.6.0-0.14.rc2.fc20 has been submitted as an update for Fedora 20.
jsoncpp-0.6.0-0.14.rc2.fc21 has been submitted as an update for Fedora 21.
jsoncpp-0.6.0-0.14.rc2.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.
jsoncpp-0.6.0-0.14.rc2.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.
Suggest updating to 0.10.0 for rawhide and f22....
(before the beta freeze, ideally!)
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