Add extern flag to avoid multiple symbol errors on Mac os X

rts/sm/BlockAlloc.h

@@ -16,7 +16,7 @@ extern void checkFreeListSanity(void);
 nat         countFreeList(void);
 #endif
 
-lnat n_alloc_blocks;   // currently allocated blocks
-lnat hw_alloc_blocks;  // high-water allocated blocks
+extern lnat n_alloc_blocks;   // currently allocated blocks
+extern lnat hw_alloc_blocks;  // high-water allocated blocks
 
 #endif /* BLOCK_ALLOC_H */

rts/sm/GC.h

@@ -39,7 +39,7 @@ extern SpinLock gc_alloc_block_sync;
 #endif
 
 #if defined(PROF_SPIN) && defined(THREADED_RTS)
-StgWord64 whitehole_spin;
+extern StgWord64 whitehole_spin;
 #endif
 
 #define WORK_UNIT_WORDS 128