Build flags sometimes are not passed to cargo

[qinsoon]

I am not familiar with Julia's build system. I am not sure if I did something that is not supposed to work, or our build script for MMTk has some issues.

I observed that I made a release build (MMTK_BUILD=release), but MMTk used for the build is a debug build, and it did not have the plan feature set (so it used the default GenImmix plan). It appeared that both MMTK_BUILD and MMTK_PLAN which were set at the build time were not effective whe cargo was invoked. So it defaulted to a debug build, and to GenImmix.

WITH_THIRD_PARTY_GC=mmtk USE_BINARYBUILDER_MMTK_JULIA=0 MMTK_MOVING=0 MMTK_PLAN=Immix MMTK_BUILD=release MMTK_JULIA_DIR=/home/yilin/Code/julia_workspace/mmtk-julia make O=julia-mmtk-immix-nonmoving

Before I observed the issue, I had a build to a different directory, and I didn't do make cleanall. I am not sure if that is related.

I also included the commit JuliaLang/julia@32aeb39, and I am not sure if this is related.