Use manifest to avoid shipping bare libstdc++-6.dll and libgcc_s_seh-1.dll in rust mingw bin dir

[vvuk]

The mingw build of rust ships with libstdc++-6.dll and libgcc_s_seh-1.dll in the bin path. This can conflict with a system (mingw/msys) installed version of those DLLs, and can cause weird errors -- for example, trying to build rust-bindgen against the system libclang.dll results in a bindgen.exe that doesn't start, because it tries to use rustc's libstdc++-6.dll, whereas libclang.dll needs a newer/different libstdc++.

A solution for this is to use manifest files and move these two DLLs to a private directory that's not in the path. In the rustc bin dir:

rustc.exe.manifest:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity
    version="1.0.0.0"
    processorArchitecture="amd64"
    name="APPLICATION"
    type="win32"
/>
<description>Rust Compiler</description>
<dependency>
    <dependentAssembly>
       <assemblyIdentity
           type="win32"
           name="Rust.PrivateRuntime"
           version="1.0.0.0"
           processorArchitecture="amd64"/>
    </dependentAssembly>
</dependency>
</assembly>

Rust.PrivateRuntime.manifest:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
    <assemblyIdentity type="win32"
      name="Rust.PrivateRuntime"
version="1.0.0.0"
processorArchitecture="amd64"/>
    <file name="rust-private\libstdc++-6.dll"/>
    <file name="rust-private\libgcc_s_seh-1.dll"/>
</assembly>

And then moving libstdc++-6.dll and libgcc_s_seh-1.dll to a new rustc/bin/rust-private/ directory.

(Note to manually test this out, you have to touch rustc.exe after creating rustc.exe.manifest, otherwise the manifest won't be read.)

[larsbergstrom]

cc @alexcrichton

[alexcrichton]

cc @brson

This would unfortunately be pretty difficult to do easily I think, but certainly not out of the question!