package: add Boehm-Demers-Weiser gc library

This is needed for applications like Zile

[Thomas:
 - use $(...) instead of ${...} to reference LIBGC_VERSION
 - add missing dependency on libatomic_ops dependencies
 - fix DEPENDANCIES -> DEPENDENCIES
 - replace += by = in dependencies definition
 - add host-pkgconf to the list of dependencies
 - fix license, it's a X11-style permissive license
 - fix license file, COPYING didn't exist, use README.md instead]

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>