Thanks for the pointer. It seems that I could use some combination of USE_PKGSRC_GCC, USE_PKGSRC_GCC_RUNTIME, and GCC_REQD=5.3 or so to rebuild everything with gcc5.
In the meantime, however, your comment about the ABI tag spurred me to realize what the problem was and instead I can compile the application with the normal pkgsrc gcc5 by defining _GLIBCXX_USE_CXX11_ABI=0. That does work, although I am still getting the following message:
/usr/bin/ld: warning: libstdc++.so.7, needed by /usr/pkg/lib/libboost_program_options.so, may conflict with libstdc++.so.6
It seems that boost, which is compiled with the native compiler, is linked against libstdc++.so.7 whereas gcc5 is linking against its own libstdc++.so.6. Can gcc5 be made to use the native library?
Thanks for your help.