On Unix, add option to pick backends

Add features 'x11'  and 'wayland' to pick backends on Linux/BSD, with
both enabled by default.

Fixes #774.

build.rs

@@ -0,0 +1,21 @@
+#[cfg(all(
+    any(
+        target_os = "linux",
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "netbsd",
+        target_os = "openbsd"
+    ),
+    not(feature = "x11"),
+    not(feature = "wayland")
+))]
+compile_error!("at least one of the \"x11\"/\"wayland\" features must be enabled");
+
+#[cfg(all(
+    target_arch = "wasm32",
+    not(feature = "web-sys"),
+    not(feature = "stdweb")
+))]
+compile_error!("at least one of the \"web-sys\"/\"stdweb\" features must be enabled");
+
+fn main() {}