Allow replacing the GSlice allocator with jemalloc

The jemalloc allocator implementation comes from the BSD world, and it's
used in various places, including:

 * Mozilla
 * Rust
 * Android
 * Cassandra
 * MariaDB
 * Redis

More info: http://jemalloc.net

Currently, support is enabled by default on Linux, but you have to opt
in using `G_SLICE=use-jemalloc`.