A more helpful error when --without-jemalloc is impossible
When jemalloc is the system allocator (on FreeBSD and NetBSD), trying to build --without-jemalloc caused an obscure compiler error. Instead, complain at configure time that --without-jemalloc cannot work. (It needs to remain an error because it is vexing when configure quietly ignores an explicit direction.)
Closes #3548 (closed)