Fails to build with LLVM_LINK_SHARED=1

[Vogtinator]

Many linux distros build LLVM as shared libraries only, so statically linking is not possible. For various reasons it's preferred to use the system LLVM instead of the bundled one, which requires that rustc is also linked dynamically against LLVM libraries.

rustc's configure has an option --enable-llvm-link-shared to do that, which internally sets the LLVM_LINK_SHARED environment variable to 1. Doing the same with mrustc's bootstrap scripts works, but only until the final link of rustc_main. I think this is because the rustc-link-lib=dylib=LLVM flag from the rustc_llvm crate's build config is not taken into account when linking the final rustc_main binary. I added a hack to minicargo/build.cpp to add -lLLVM when building rustc_main and the binary is built successfully that way.