* configure.in: add --enable-dynamic option for linking our local binaries

* configure.in: add --enable-dynamic option for linking our local binaries
  with the shared libraries instead of static, defaults to no
* build/lib-shared.mk: if LINK_BINS_DYNAMIC is set we create a symlink to
  the .so and .so.# file along with the .a and .la files
* build/lib.mk: make sure the above links get removed on clean target
* build/top.mk: add define for LINK_BINS_DYNAMIC
* tests/scripts/defines.sh: add export for LD_LIBRARY_PATH so that tests will
  run without requiring installation of libraries when we use --enable-dynamic
  (LD_LIBRARY_PATH is always set, since it can't really hurt).