Add a default-on "use_std" feature
This adds a `use_std` Cargo feature which disables `#![no_std]` builds of libc,
but is enabled by default. The library will currently continue to link to the
standard library to maintain backwards compatibility with the 0.2 series and
older Rust compilers for now, but this default can possible be changed in the
future.
diff --git a/Cargo.toml b/Cargo.toml
index 067d078..28e5b33 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,4 +14,5 @@
"""
[features]
-default = []
+default = ["use_std"]
+use_std = []