bug#23974: Outdated instructions regarding bdw-gc in guile 2.0.11

[Ambrose LI]

When building guile 2.0.11, configure fails with a "No package
'bdw-gc' found" error, and README claims that "bdw-gc" (confusingly
referred to as "libgc") is still "available" at
http://www.hpl.hp.com/personal/Hans_Boehm/gc/ . However, the garbage
collector has long disappeared from HP's home page (at least since
several years ago, definitely before 2014 when guile 2.0.11 was
released).

The "bdw-gc" *appears* to be now located at
https://github.com/ivmai/bdwgc . If this is the correct location the
README should at least be updated. (The instructions should also at
least be made consistent so that the gc is not called "bdw-gc" in one
place and "libgc" in another.)