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Force libtool to static link the PIC version of our jemalloc import into
libaml, making libaml standalone. This requires us to test some
additional libraries in our own configure (pthread, and dlopen).

This also solves the long-standing issue of `make check` only working after
`make install`, while removing our custom jemalloc from the installed
libraries.

Fixes #26.

The jemalloc subpackage install rule is too big, and includes stuff that
we don't need to add to the target system. This patch makes sure we only
install the valuable stuff.

We also correct the pkg-config data to match the new installs.

This is the first working version of the library. It does very little:
- only numa support
- one allocation per node only
- limited tests
- limited set of functions

Nevertheless, this gives a good idea of what the API should look like, and the
kind of benchmarks we can write with it.