Correctly choose profile to use.

If, e.g., a user sets `CARGO_PROFILE_RELEASE_DEBUG=1`, the `DEBUG`
environment variable will be set accordingly, and so the debug build
will be used.

Instead, we should use the PROFILE env variable (see
https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts)
to determine which profile to use.

This isn't 100% correct, because it won't pass through any other flags
(in this case, we won't have debug information for the cubeb backend),
but it is more correct to ensure the matching profile.

cubeb-sys/build.rs

@@ -113,7 +113,7 @@ fn main() {
         .env("CARGO_BUILD_TARGET", &target)
         .build();
 
-    let debug = env::var("DEBUG").unwrap().parse::<bool>().unwrap();
+    let debug = env::var("PROFILE").unwrap() == "debug";
 
     println!("cargo:rustc-link-lib=static=cubeb");
     if windows {