Unix configure: add --with-libjson-c configure option to build against external libjson-c (>= 0.11) (#4676)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26293 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx