build: back-port some spec changes from recent RHEL builds
build: back-port some spec changes from recent RHEL builds

Reduce the propogation of explicit version numbers throughout
the fedora spec file, trying to keep 'em all together and use
consistently-named macros elsewhere.  Remove build dependency
on json packages, switch this on/off based on platform version
instead.

aeg-aeg/pcpfans,tjanez/pcp,edwardt/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,tjanez/pcp,prasincs/pcp,tjanez/pcp,aeg-aeg/pcpfans,adfernandes/pcp,prasincs/pcp,edwardt/pcp,wuliming/pcp,tjanez/pcp,edwardt/pcp,edwardt/pcp,tjanez/pcp,aeg-aeg/pcpfans,wuliming/pcp,aeg-aeg/pcpfans,prasincs/pcp,adfernandes/pcp,prasincs/pcp,tjanez/pcp,tjanez/pcp,edwardt/pcp,prasincs/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,prasincs/pcp,wuliming/pcp,adfernandes/pcp,edwardt/pcp,edwardt/pcp,prasincs/pcp,prasincs/pcp,edwardt/pcp,adfernandes/pcp,tjanez/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans