Add DEBUG constexpr

Enable checking use of a constexpr rather than ifdef for checking if in
DEBUG.

src/pal/pal_apple.h

@@ -113,9 +113,8 @@ namespace snmalloc
     {
       SNMALLOC_ASSERT(is_aligned_block<page_size>(p, size));
 
-#  if !defined(NDEBUG)
-      memset(p, 0x5a, size);
-#  endif
+      if constexpr (DEBUG)
+        memset(p, 0x5a, size);
 
       // `MADV_FREE_REUSABLE` can only be applied to writable pages,
       // otherwise it's an error.

src/pal/pal_bsd.h

@@ -35,9 +35,9 @@ namespace snmalloc
     {
       SNMALLOC_ASSERT(is_aligned_block<OS::page_size>(p, size));
 
-#if !defined(NDEBUG)
-      memset(p, 0x5a, size);
-#endif
+      if constexpr (DEBUG)
+        memset(p, 0x5a, size);
+
       madvise(p, size, MADV_FREE);
 
       if constexpr (PalEnforceAccess)

src/pal/pal_linux.h

@@ -70,11 +70,11 @@ namespace snmalloc
 
       if constexpr (PalEnforceAccess)
       {
-#  if !defined(NDEBUG)
         // Fill memory so that when we switch the pages back on we don't make
         // assumptions on the content.
-        memset(p, 0x5a, size);
-#  endif
+        if constexpr (DEBUG)
+          memset(p, 0x5a, size);
+
         madvise(p, size, MADV_FREE);
         mprotect(p, size, PROT_NONE);
       }

src/pal/pal_posix.h

@@ -189,11 +189,11 @@ namespace snmalloc
 
       if constexpr (PalEnforceAccess)
       {
-#if !defined(NDEBUG)
         // Fill memory so that when we switch the pages back on we don't make
         // assumptions on the content.
-        memset(p, 0x5a, size);
-#endif
+        if constexpr (DEBUG)
+          memset(p, 0x5a, size);
+
         mprotect(p, size, PROT_NONE);
       }
       else