build: enable parallel building

build: enable parallel building

$ make modules -j8
cd kernel; make -C /lib/modules/`uname -r`/build M=`pwd` V=0 \
                        IP_SET_MAX=256 \
                        NETLINK_DUMP_CONST=const \
                        NFNL_CB_CONST=const modules
make[1]: Entering directory "/usr/src/linux-2.6.36-rc8-34-obj/x86_64/default"
make[1]: warning: jobserver unavailable: using -j1.
Add '+' to parent make rule.

The "fix" here is that we have to use ${MAKE} instead of make.
(NB: The extra cd is redundant and can be appended to M=.)