Added shared library support to build system.

Details:
- Modified top-level Makefile to support building shared (dynamic)
  libraries.
- Updated most configurations' make_defs.mk files to include necessary
  compiler/linker flags needed by top-level Makefile.
- Note that by default, all configurations presently do NOT build
  shared libraries. To enable, one must change the value of
  BLIS_ENABLE_DYNAMIC_BUILD to 'yes'.