allow external jemalloc?

[0-wiz-0]

NetBSD comes with jemalloc as its default allocator. I didn't find a way to make the jemallocator-sys crate use an external jemalloc, so this unnecessarily compiles jemalloc again.

Also, for packaging systems in general, there might already be a separate jemalloc package.

Would you consider supporting the use of an existing jemalloc installation?

[BusyJay]

Note sometimes we backport patches from jemalloc/jemalloc to tikv/jemalloc that is not released in time. In such case, jemalloc from NetBSD may not match the version jeamallocator expects.

Giving this pitfall, I still think it's OK to allow accept a third party libjemalloc.a or libjemalloc.so as long as it's controlled via a feature gate.