Find libunwind include path using pkg-config, fixes rust-lang#102

Author: valpackett

backtrace-sys/Cargo.toml

@@ -16,3 +16,4 @@ libc = "0.2"
 
 [build-dependencies]
 cc = "1.0"
+pkg-config = "0.3"

backtrace-sys/build.rs

@@ -1,3 +1,4 @@
+extern crate pkg_config;
 extern crate cc;
 
 use std::env;
@@ -48,6 +49,12 @@ fn main() {
         }
     }
 
+    if let Ok(unwind) = pkg_config::Config::new().probe("libunwind") {
+        for path in unwind.include_paths {
+            build.include(path);
+        }
+    }
+
     File::create(out_dir.join("backtrace-supported.h")).unwrap();
     build.define("BACKTRACE_SUPPORTED", "1");
     build.define("BACKTRACE_USES_MALLOC", "1");