More standard ways of linking other allocators - Correctly apply tc-malloc recommended flags.

Changes are:

• Correctly cancel the built-ins to malloc and friends that GCC and friends use when compiling with an alternative allocator library.
• Add the option to link in TBBmalloc following the same pattern as tcmalloc and jemalloc.
• Make sure we do not have more than one alternative allocator library linked in.
• Update the README.
• Also make use of the alternative allocators in the unit tests.
• Print the allocator choice into the snapshot meta-data.