Fix undefined reference caused by wrong detection of gcc builtin atomic functions.

I found the current detection is not correct and finally got following errors on my linux build:
```
xbmc/threads/threads.a(Atomics.o): In function `AtomicSubtract(long volatile*, long)':
/var5/xbmc/xbmc/threads/Atomics.cpp:386: undefined reference to `__sync_sub_and_fetch_4'
xbmc/threads/threads.a(Atomics.o): In function `AtomicDecrement(long volatile*)':
/var5/xbmc/xbmc/threads/Atomics.cpp:308: undefined reference to `__sync_sub_and_fetch_4'
xbmc/threads/threads.a(Atomics.o): In function `AtomicAdd(long volatile*, long)':
/var5/xbmc/xbmc/threads/Atomics.cpp:230: undefined reference to `__sync_add_and_fetch_4'
xbmc/threads/threads.a(Atomics.o): In function `AtomicIncrement(long volatile*)':
/var5/xbmc/xbmc/threads/Atomics.cpp:152: undefined reference to `__sync_add_and_fetch_4'
xbmc/threads/threads.a(Atomics.o): In function `cas(long volatile*, long, long)':
/var5/xbmc/xbmc/threads/Atomics.cpp:30: undefined reference to `__sync_val_compare_and_swap_4'
/var5/xbmc/xbmc-deps/i686-linux-gnu/lib/libtag.a(mpegheader.cpp.o): In function `TagLib::RefCounter::deref()':
mpegheader.cpp:(.text._ZN6TagLib10RefCounter5derefEv[TagLib::RefCounter::deref()]+0x15): undefined reference to `__sync_sub_and_fetch_4'
collect2: ld returned 1 exit status
[btfans@localhost xbmc]$ vi t.c

```

so I found how to fix it, reference: http://osgeo-org.1560.x6.nabble.com/undefined-reference-to-sync-sub-and-fetch-4-td5009362.html