Fix Makefile to properly include zvm.h

[larsbutler]

At the moment, we need to set CPATH=/usr/x86_64-nacl/include to build toolchain. This fix should probably be moved to the Makefile, per this comment: #14 (comment)

[pkit]

zvm.h is used by ZRT only (in the toolchain build)
I suppose Makefile or Makefile.env can be altered to use more proper paths.
Do all Linux distros use /usr as syspath?

[mgeisler]

Thanks @larsbutler for recording my comment as an issue — I should have done that directly...

@pkit I think the zrt Makefile.env should respect PREFIX instead of ZVM_PREFIX as I think it does today. That is, I see it doing:

ZVM_PREFIX_ABSPATH = $(abspath $(ZVM_PREFIX))
ZRT_INCLUDE_PATH= -I${ZVM_PREFIX_ABSPATH}/api \
-I${ZVM_PREFIX_ABSPATH}/x86_64-nacl/include \
-I${ZRT_ROOT}/lib \
# ...

The PREFIX variable isn't used and that's what the tools will set when building .deb and .rpm packages. We can of course also update the build tools to explicitly set ZVM_PREFIX.