It seems like path issue. FindRGW is basically looking for rados/rgw_file.h and reading the version number from there. There should be a message "FindRGW: using PATH HINT:". You may need to set the path to correct location where librgw-devel is installing rados/rgw_flie.h. In my environment it is at /usr/include/rados. Also, you should ensure that you have compiling with right version of librgw.