boost-1.66.0-0.1.fc28 builds binaries /usr/bin/bjam, /usr/bin/quickbook which do not use the full set of compiler/linker flags from redhat-rpm-config. I haven't tried to find the root case, not daring to look at the Boost build system. Compiler flags are currently available through %{optflags}/$RPM_OPT_FLAGS, and linker flags are available in the shell variable $RPM_LD_FLAGS.
This should be fixed in boost-1.66.0-2.fc28 which is building now.