Tweaks to get the building of the native library working properly:
1) Since autoconf abhors using GNU Make's :=, build "evaluate once" variables in configure.ac.
2) Require dlopen support.
3) Add logic for statically figuring out our library extension based on configure host_os.
4) Break out buliding of the JAR Manifest
5) Create our own dependency for the static library so as to avoid possible race conditions.