Fix build when using clang and libunwind is installed

[Aatch]

This fixes issue #5641

[brson]

@graydon How do you feel about this change to configure? This is the first such test for a header file.

[brson]

Regardless of whether configure detects that unwind.h exists our source code still contains #include <unwind.h>. So unwind.h has to exist or this isn't going to build. Maybe we should not be redefining the UnwindAction and UnwindException types at all.

[thestinger]

@brson: gcc and clang both come with their own unwind.h, so that's what gets used if libunwind isn't installed. Rust seems to require gcc extensions that clang doesn't expose in the header - it ends up working because Rust links against libstdc++/libgcc_s but it might not work with libc++/libc++abi.

[graydon]

I am not sure this is the right approach. If libunwind is a requirement on platforms where it works (and it is, right?) then surely we should be detecting and failing during configure, not detecting and compiling a broken binary, no?

[thestinger]

Clang doesn't expose the full unwinding interface in the fallback unwind.h but it still seems to work thanks to linking against libstdc++. I don't think Rust ever links against libunwind.

[graydon]

What do we need from unwind.h if we don't link against it? Maybe we can just remove the dependency?

[brson]

Yeah, we only need a few declarations just to pass some values through _UnwindResume. Maybe we can forget about unwind.h completely and use our own typedefs.

[Aatch]

Hey, so I just removed all the imports from rust_unwind.h, it compiles and links though I haven't run any tests yet.

I just copied the declarations of the types that upcall uses and it seems to work at the moment. Since this both solves the problem and removes the dependency on (lib)unwind.h I think it works.

I'll push the commit to the branch when everything has finished compiling.

[brson]

@Aatch thanks!

[Aatch]

@bors still hasn't picked this up, is there a way that I can get the build bots to do it myself?