openswan: fix possible build failure

openswan's Makefile uses gcc instead of $(CC) for gcc version detection
to use advanced warning/error options. The problem is that if the
host gcc version is newish (>=4.6) and the gcc for the target is not it
uses unsupported options.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>