Modify Makefile.PL to set UNINST=1 if needed on old perls

Because ExtUtils::MakeMaker bundles JSON-PP and might install it into the core library path on Perls before 5.12, upgrades to JSON::PP will not be seen UNINST=1 to remove to old JSON::PP.  This commit adds conditional code to Makefile.PL to do that.  Similar patches are being applied to other modules bundled by EU::MM.