Build libbacktrace with mmapio.c instead of read.c

[Aaron1011]

Fixes #289

mmap should be available on all platforms we support, so we can use
libbacktrace's more efficient mmap-based code.

[alexcrichton]

It looks like Windows may need to be exempted and continue to use the older implementation?

[Aaron1011]

@alexcrichton: At this point, we might want to just switch to using the configure script from libbacktrace, so that we don't need to maintain our own hacky mmap detection.

Do you have a strong preference for the current approach of hardcoding the relevant paths into build.rs?

[alexcrichton]

Having maintained running the configure script for years, yes I have quite a strong preference for never going back to that. It's ok if you label it "hacky" but I would personally label it "vastly more reliable than anything we had before" from my experience.

I would be fine with writing down "if windows compile this file, else that file"

[Aaron1011]

@alexcrichton: Fair enough - I'll add an explicit check for Windows.

[Aaron1011]

@alexcrichton: Updated

[alexcrichton]

Thanks!