After a successful "make install", subsequent "make install" runs fail trying to rmdir a symlink

This PR fixes this issue:

rmdir: failed to remove `/home/jmaurice/Development/installdir/json': Not a directory
make[3]: *** [install-data-hook] Error 1
make[3]: Leaving directory `/home/jmaurice/Development/json-c'
make[2]: *** [install-data-am] Error 2
make[2]: Leaving directory `/home/jmaurice/Development/json-c'
make[1]: *** [install-am] Error 2
make[1]: Leaving directory `/home/jmaurice/Development/json-c'
make: *** [install-recursive] Error 1