Make some small Mac-specific modifications to V8 to make it work on MacOS X 1...

Make some small Mac-specific modifications to V8 to make it work on MacOS X 10.4
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.

Committed: http://code.google.com/p/v8/source/detail?r=2213

[iposva, 2009-06-18 00:19:23]

TBR=ager

[Mads Ager (chromium), 2009-06-18 05:27:02]

LGTM