Turn off -Wextern-c-compat for libunwind.h since it fails that check.

This doesn't surface if you install libunwind globally since clang will
ignore errors from system libraries; this change should fix it for people
who are installing libunwind locally (as is the goal).
Fixes #27

src/runtime/stacktrace.cpp

@@ -13,8 +13,18 @@
 // limitations under the License.
 
 #define UNW_LOCAL_ONLY
+
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wextern-c-compat"
+#endif
+
 #include <libunwind.h>
 
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+
 
 #include "core/options.h"