package/gauche: fix getcontext build issue at link time

This patch is ported from bdwgc package. Since Gauche uses an internal
copy of the boehm gc code, it is affected by the same problem.

Both configure and configure.ac are modified because autoreconf fails
due to an incompatibility with the version of the autotools used by
Buildroot.

Fixes:

   http://autobuild.buildroot.org/results/8c6/8c6ce526735a36b8c8b4c6047e3c69039c4527cb/
   http://autobuild.buildroot.org/results/894/89460829a737a4dab19103f7a46905636420b0a8/



[Thomas: adjust commit log and patch description.]

Signed-off-by: Hiroshi Kawashima <kei-k@ca2.so-net.ne.jp>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>