bdwgc: add patch automatically detecting for NO_GETCONTEXT need

This patch is necessary to successfully build guile with toolchain
missing support for getcontext.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: add a comment, change variable name, use
 AS_IF, remove debug traces, use AC_CHECK_FUNCS (Thomas)]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>