allow custom PREFIX and MULTILIB in bulding TvmJIT

`make TVMJIT_PREFIX=/usr` will pass PREFIX to TvmJIT Makefile.

`make TVMJIT_MULTILIB="lib/$(gcc -print-multiarch)"` will pass MULTILIB to
TvmJIT Makefile.

It's useful for building Shine with default package.path and package.cpath
values mimicking the same default values of LuaJIT installed via way preferred
by Linux distro (like LuaJIT packaged in Debian and installed via `apt`).

Makefile

@@ -136,9 +136,11 @@ ${BUILD}/deps/liblpeg.a: ${LPEG}
 ${BUILD}/deps/libtvmjit.a: ${TJ}
 	cp ${TVMDIR}/libtvmjit.a ${BUILD}/deps/libtvmjit.a
 
+${TJ}: TVMJIT_PREFIX ?= ${PREFIX}
+${TJ}: TVMJIT_MULTILIB ?=
 ${TJ}:
 	git submodule update --init ${DEPDIR}/tvmjit
-	${MAKE} PREFIX=${BUILD} TRAVIS=1 -C ${DEPDIR}/tvmjit
+	${MAKE} PREFIX=${TVMJIT_PREFIX} MULTILIB=${TVMJIT_MULTILIB} TRAVIS=1 -C ${DEPDIR}/tvmjit
 
 ${LPEG}:
 	make -C ${DEPDIR}/lpeg ${LPEG_BUILD}
@@ -175,6 +177,7 @@ realclean: clean
 	stat ${DEPDIR}/nanomsg/Makefile && make -C ${DEPDIR}/nanomsg clean
 	rm -rf ${BUILD}
 
+bootstrap: TVMJIT_PREFIX = ${BUILD}
 bootstrap: ${TJ} ${LPEG}
 	mkdir -p boot/bin
 	mkdir -p boot/lib