examples: use SPDX identifiers

The SPDX identifiers [1] are a legally binding shorthand, which can be
used instead of the full boiler plate text. This is another step towards
implementing the full REUSE spec [2] to help with copyright and
licensing audits and compliance.

This will reduce a lot a manual work required for the licensing audit
required in Debian on each update.

Relicense all examples from 'Boehm-GC' to the more well-known and
functionnaly identical 'MIT' license. This is possible since all the
examples were written by Mathieu Desnoyers and only a few trivial fixes
from external contributors were applied over the years.

[1] https://spdx.org/ids-how
[2] https://reuse.software/tutorial/

Change-Id: I052ab4fca0f8979113f3bae3b78982eb30f50c58
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>