clang does not like rust_box's flexible array

[erickt]

Compiling rust_builtin.cpp errors out with this message:

compile: rt/rust_builtin.o
../rust/src/rt/rust_builtin.cpp:388:13: error: flexible array 'data' not allowed in otherwise empty struct
    uint8_t data[];
            ^

[erickt]

The problem stemmed from an issue with the build system. I need to configure with CFG_LLVM_CONFIG and CFG_CONFIG set, but when Makefile.in changes, it triggers a reconfigure, which blows away my configuration saved in config.mk. This causes make to fall back onto the clang 2.9svn, which results in the error above. I'll file a new bug to have reconfiguring preserve the CFG_* settings.