Add basic Darwin support to libcorkscrew.

This might not seem like much, but it's already better than Mac OS'
backtrace(3).

(cherry picked from commit 2a46f6293f19b9d6100187fbd53088088e671b88)

Change-Id: I197b06c4c19f728ad6f194b2f473bd9dcd99b8a6
diff --git a/libcorkscrew/demangle.c b/libcorkscrew/demangle.c
index 54247cb..30ab1b0 100644
--- a/libcorkscrew/demangle.c
+++ b/libcorkscrew/demangle.c
@@ -25,6 +25,12 @@
                              int *status);
 
 char* demangle_symbol_name(const char* name) {
+#if defined(__APPLE__)
+    // Mac OS' __cxa_demangle demangles "f" as "float"; last tested on 10.7.
+    if (name != NULL && name[0] != '_') {
+        return NULL;
+    }
+#endif
     // __cxa_demangle handles NULL by returning NULL
     return __cxa_demangle(name, 0, 0, 0);
 }