With much help from joerg, it was found that somehow c++ is pulling in the
system evp.h from /usr/include/openssl rather than the one from pkgsrc in
/usr/pkg/include/openssl, even though on the command line
-I/usr/pkgsrc/wip/libzrtpcpp/work/.buildlink/include/openssl points to
/usr/pkg/include/openssl and appears on the command line before the other -I
flags. Adding -save-temps to CPPFLAGS shows the system evp.h being parsed in
hmac256.ii. Why is this happening? I don't understand.