Set use-jemalloc conditionally instead of ignoring it for some targets

[sertonix]

src/main.rs and Cargo.toml include a complex expression to not use jemalloc on some targets even when use-jemalloc has been set explicitly. As far as I can tell most of the cases where jemalloc is disabled is due to build failures. Since cargo can be configured to use a system provided jemalloc these build failures can be avoided on some targets (in my case on 32-bit musl and riscv64). Therefor it would be nice to conditionally have use-jemalloc in the default features instead of force disabling it on some targets.

[tmccombs]

Unfortunately, it doesn't look like it is currently possible to enable features based on the target platform, see rust-lang/cargo#1197

So I'm not sure how to do this without requiring specifying the jemalloc feature on all platforms.

[sertonix]

Thanks for taking a look! In that case there isn't really something that can be done on the fd side.