pkg-perl: fix build of perl XS packages

Set OPTIMIZE in order to avoid the use of the host $Config{optimize} value
which could cause breakage when cross-compiling for the target.

See discussion in http://lists.busybox.net/pipermail/buildroot/2014-August/104129.html

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>