Add XPI for OSX

/!\ Some trouble with the official build
This XPI was make with my own build of xulrunner-sdk
I try this xpi with nokia webcl sample seems working

.mozconfig use for my build

mk_add_options MOZ_CO_PROJECT=xulrunner
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-xul-mac-res
mk_add_options MOZ_MAKE_FLAGS="-s -j8"

ac_add_options --disable-tests
#ac_add_options --enable-logging
#ac_add_options --enable-debug
#ac_add_options --disable-optimize
ac_add_options --disable-crashreporter
ac_add_options --disable-installer
ac_add_options --enable-application=xulrunner
ac_add_options --with-macos-sdk=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk
ac_add_options --enable-macos-target=10.8

CC=clang
CXX=clang++

RANLIB=ranlib
AR=ar
AS=$CC
LD=ld
STRIP="strip -x -S"
CROSS_COMPILE=1