trinity: use Boehm-Demers-Weiser's garbage collecting memory allocator

trinity currently quite often does not care about freeing memory on
its own that was fetched through malloc(). Therefore it can happen if
trinity runs for quite a long time, that oom-killer gets invoked. So
for now use the Boehm-Demers-Weiser's garbage collecting memory
allocator as a malloc replacement to avoid such situations.

In this patch, we wrap all BDW functions into bdw_* equivalents, so that
we could later on still #ifdef something if we want to and replace all
calls based on the build. Calling bdw_free() is still legimite, but the
memory is reclaimed via bdw_gcollect(), which would solve trinity's
memory management.

Dependency that we need when building (in case people don't want
additional libs, follow-up patches could still #ifdef bdw_* wrappers
to the glibc functions):

  Fedora: yum install gc-devel

  Debian: apt-get install libgc-dev

More information on Boehm-Demers-Weiser allocator can be found here:

  http://www.hpl.hp.com/personal/Hans_Boehm/gc/

Note that it is said that the performance is competitive with malloc(3),
free(3) calls.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>