Use gimli on Linux, coresym on macOS

Fixes source printing after switching the working dir to something != the root source directory by ditching libbacktrace.

Resolves #2

Cargo.toml

@@ -16,14 +16,19 @@ keywords = [
     "pretty",
 ]
 
+[features]
+failure-bt = ["failure"]
+
 [dependencies]
-backtrace = "0.3"
 term = "0.5"
 atty = "0.2"
+failure = { version = "0.1", optional = true }
 
-[features]
-failure-bt = ["failure"]
+[target.'cfg(target_os="linux")'.dependencies]
+backtrace = { version = "0.3", features = ["gimli-symbolize"] }
+
+[target.'cfg(target_os="macos")'.dependencies]
+backtrace = { version = "0.3", features = ["coresymbolication"] }
 
-[dependencies.failure]
-optional = true
-version = "0.1.5"
+[target.'cfg(not(any(target_os="macos", target_os="linux")))'.dependencies]
+backtrace = { version = "0.3" }