Fix: `SpinLock` not defined

rts/include/rts/storage/HeapAlloc.h

@@ -8,6 +8,8 @@
 
 #pragma once
 
+#if !defined(CMINUSMINUS)
+
 #if defined(THREADED_RTS)
 // needed for HEAP_ALLOCED below
 extern SpinLock gc_alloc_block_sync;
@@ -16,6 +18,8 @@ extern SpinLock gc_alloc_block_sync;
 #define ACQUIRE_ALLOC_BLOCK_SPIN_LOCK() ACQUIRE_SPIN_LOCK(&gc_alloc_block_sync)
 #define RELEASE_ALLOC_BLOCK_SPIN_LOCK() RELEASE_SPIN_LOCK(&gc_alloc_block_sync)
 
+#endif // !defined(CMINUSMINUS)
+
 /* -----------------------------------------------------------------------------
    The HEAP_ALLOCED() test.