bdwgc: Add new package

bdwgc: Add new package

bdwgc is a garbage collector for C and C++. The Boehm-Demers-Weiser
conservative garbage collector can be used as a garbage collecting
replacement for C 'malloc' or C++ 'new'.

[Thomas:
 - drop the dependency on host-bdwgc, since it is not needed. Only
   host-guile will need host-bdwgc, but bdwgc itself doesn't need
   host-bdwgc.
 - add dependency on host-pkgconf, needed to detect libatomic_ops
 - add --with-libatomic-ops=yes to force bdwgc to use the system
   libatomic_ops, and not the internal one.
 - instead of using BDWGC_LIBTOOL_PATCH = NO (needed to avoid patching
   libtool because the libtool version used by bdwgc does not have a
   corresponding patch in Buildroot), use BDWGC_AUTORECONF = YES,
   which generates a proper libtool (without this, some -L/usr/lib
   flags are added by libtool).]

Signed-off-by: Pedro Aguilar <paguilar@paguilar.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>