Issue on Debian 7 (wheezy).

I ran into this when trying to run my own build of `alacritty`. I verified that my `/usr/lib/x86_64-linux-gnu/libXrandr.so.2.2.0` does not export `XRRChangeProviderProperty`.

```
thread 'main' panicked at 'Failed to initialize any backend!
Wayland status: NoWaylandLib
X11 status: LibraryOpenError(
OpenError {
kind: Symbol,
detail: "XRRChangeProviderProperty - /usr/lib/x86_64-linux-gnu/libXrandr.so.2.2.0: undefined symbol: XRRChangeProviderProperty"
}
)
', /opt/rust/.cargo.Linux/registry/src/github.com-1ecc6299db9ec823/winit-0.15.0/src/platform/linux/mod.rs:418:9
stack backtrace:
0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
1: std::panicking::default_hook::{{closure}}
at libstd/sys_common/backtrace.rs:71
2: std::panicking::rust_panic_with_hook
at libstd/panicking.rs:227
at libstd/panicking.rs:463
3: std::panicking::begin_panic
at /checkout/src/libstd/panicking.rs:397
4: alacritty::display::Display::new
at /opt/rust/.cargo.Linux/registry/src/github.com-1ecc6299db9ec823/winit-0.15.0/src/platform/linux/mod.rs:418
5: alacritty::main
at src/main.rs:104
at src/main.rs:62
6: std::rt::lang_start::{{closure}}
at /checkout/src/libstd/rt.rs:74
7: main
8: __libc_start_main
9:
```

Can this symbol being missing be marked as non-fatal somehow?