The issue here is that RTLD_NEXT is not defined by the posix standard . So the GNU people don’t enable it unless you #define _GNU_SOURCE or -D_GNU_SOURCE.
Other relevant pieces of POSIX are dlfcn.h and dlsym.h. Interestingly, the later mentions RTLD_NEXT. Apparently, the GNU people are a bit confused about what is an extension and what is not.