CPP macro usage in TArray relies on undefined behavior

[bgamari]

According to clang, the CPP guards in #66 rely on undefined behavior:

/private/var/lib/gitlab-runner/builds/7KQsmDei/0/ghc/ghc/libraries/stm/Control/Concurrent/STM/TArray.hs:9:5: error:
     error: macro expansion producing 'defined' has undefined behavior [-Werror,-Wexpansion-to-defined]
  |
9 | #if HAS_UNLIFTED_ARRAY
  |     ^
#if HAS_UNLIFTED_ARRAY
    ^
/private/var/lib/gitlab-runner/builds/7KQsmDei/0/ghc/ghc/libraries/stm/Control/Concurrent/STM/TArray.hs:7:28: error:
     note: expanded from macro 'HAS_UNLIFTED_ARRAY'
  |
7 | #define HAS_UNLIFTED_ARRAY defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 904
  |                            ^
#define HAS_UNLIFTED_ARRAY defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 904
                           ^