under openbsd, the library path of libstdc++ need to be explicit (due
to the fact the default linker `cc` is gcc-4.2, and not gcc-4.9).
but when a recent LLVM is installed, rustc compilation pikes the bad
LLVM version (which live in /usr/local/lib, which is same directory of
libestdc++.so for gcc-4.9).
this patch move the libstdc++ path from RUST_FLAGS_<target> to special
variable, and use it *after* LLVM_LIBDIR_RUSTFLAGS_<target> in
arguments.
r? @alexcrichton
4bd302ee9f