Use dladdr to get symbols for environments which doesn't use ELF and

Use dladdr to get symbols for environments which doesn't use ELF and
has execinfo.h (e.g., MacOSX 10.5). Though dladdr may not be async
signal safe, it's OK since glog's stacktrace doesn't depend on signals.

git-svn-id: https://google-glog.googlecode.com/svn/trunk@7 eb4d4688-79bd-11dd-afb4-1d65580434c0