Add debug_in_debug feature

This new feature is similar to  the `debug` feature with the slight change that it is enabled automatically for debug configurations.

Author: maxded

Cargo.toml

@@ -28,4 +28,5 @@ default = ["secure"]
 secure = ["libmimalloc-sys/secure"]
 override = ["libmimalloc-sys/override"]
 debug = ["libmimalloc-sys/debug"]
+debug_in_debug = ["libmimalloc-sys/debug_in_debug"]
 local_dynamic_tls = ["libmimalloc-sys/local_dynamic_tls"]

libmimalloc-sys/Cargo.toml

@@ -19,6 +19,7 @@ cc = "1.0"
 [features]
 secure = []
 debug = []
+debug_in_debug = []
 override = []
 extended = ["cty"]
 local_dynamic_tls = []

libmimalloc-sys/build.rs

@@ -32,7 +32,9 @@ fn main() {
         }
     }
 
-    if env::var_os("CARGO_FEATURE_DEBUG").is_some() {
+    if env::var_os("CARGO_FEATURE_DEBUG").is_some()
+        || (env::var_os("CARGO_FEATURE_DEBUG_IN_DEBUG").is_some() && cfg!(debug_assertions))
+    {
         build.define("MI_DEBUG", "3");
         build.define("MI_SHOW_ERRORS", "1");
     } else {