libiscsi: Force configure to use gcc for LD

Fixes
http://autobuild.buildroot.org/results/0a9/0a96f3654983ed2efc851522352dec43c1df0077/



[Thomas:
  - pass the CC variable in <pkg>_CONF_ENV instead of <pkg>_CONF_OPTS
  - add comment in the code explaining why we are doing this.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>