rather than just 10.5 and up.
1: Set the right compile flags and predefines to get backward compatible Unix
system calls.
2: Explicitly weak import the functions in execinfo.h and check at runtime to
see if that library loaded before calling backtrace(). 

Original change submitted by maf@google.com and reviewed at http://codereview.chromium.org/126241.
Review URL: http://codereview.chromium.org/132002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2213 ce2b1a6d-e550-0410-aec6-3dcde31c8c00