This is not unspecified behavior. The warning of clang is wrong. Your code is legal c++. The Standard (C++11) states, that cv-qualifiers on top of a function type are ignored.
Hence, it doesn't make any sense to put a const qualifier on top of a function type. If you want your pointer to be const then write
junky_t * const junk;
otherwise just write
junky_t * junk;