build with system libs if libyajl2-gem missing libs

libyajl2-gem with USE_SYSTEM_LIBYAJL2=1 is just missing the libs
and includes in the path, so if we can't find libyajl2 after setting
up the libyajl2-gem, then go hunting for libyajl2 in the system
and/or use the --with-yajl-include/dir/lib options