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.

examples/Makefile.am

@@ -24,7 +24,7 @@ AM_CFLAGS = -I$(top_srcdir)/src $(HDF5_CPPFLAGS) $(GSL_INCS) $(FFTW_INCS)
 AM_LDFLAGS = $(HDF5_LDFLAGS)
 
 # Extra libraries.
-EXTRA_LIBS = $(HDF5_LIBS) $(FFTW_LIBS) $(PROFILER_LIBS) $(TCMALLOC_LIBS) $(JEMALLOC_LIBS) $(GRACKLE_LIBS) $(GSL_LIBS)
+EXTRA_LIBS = $(HDF5_LIBS) $(FFTW_LIBS) $(PROFILER_LIBS) $(TCMALLOC_LIBS) $(JEMALLOC_LIBS) $(TBBMALLOC_LIBS) $(GRACKLE_LIBS) $(GSL_LIBS)
 
 # MPI libraries.
 MPI_LIBS = $(METIS_LIBS) $(MPI_THREAD_LIBS)