Update libbacktrace to latest master (#299)

* Update libbacktrace to latest master

* Temporarily switch to t6/libbacktrace which has 5d4a5ff0d17040b5dd5dfaf2c3694b9a7b67b85d

* Define HAVE_MACH_O_DYLD_H on macOS

Author: Tobias Kortkamp

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "backtrace-sys/src/libbacktrace"]
 	path = crates/backtrace-sys/src/libbacktrace
-	url = https://github.com/rust-lang-nursery/libbacktrace
+	url = https://github.com/t6/libbacktrace

crates/backtrace-sys/build.rs

@@ -49,6 +49,7 @@ fn main() {
 
     if target.contains("darwin") {
         build.file("src/libbacktrace/macho.c");
+        build.define("HAVE_MACH_O_DYLD_H", "1");
     } else if target.contains("windows") {
         build.file("src/libbacktrace/pecoff.c");
     } else {