Make some feature flag defaults overridable in the Limited-API case.

Cython/Utility/ModuleSetupCode.c

@@ -163,12 +163,14 @@
   #define CYTHON_USE_PYLIST_INTERNALS 0
   #undef CYTHON_USE_UNICODE_INTERNALS
   #define CYTHON_USE_UNICODE_INTERNALS 0
-  #undef CYTHON_USE_UNICODE_WRITER
-  #define CYTHON_USE_UNICODE_WRITER 1
+  #ifndef CYTHON_USE_UNICODE_WRITER
+    #define CYTHON_USE_UNICODE_WRITER 1
+  #endif
   #undef CYTHON_USE_PYLONG_INTERNALS
   #define CYTHON_USE_PYLONG_INTERNALS 0
-  #undef CYTHON_AVOID_BORROWED_REFS
-  #define CYTHON_AVOID_BORROWED_REFS 0
+  #ifndef CYTHON_AVOID_BORROWED_REFS
+    #define CYTHON_AVOID_BORROWED_REFS 0
+  #endif
   #undef CYTHON_ASSUME_SAFE_MACROS
   #define CYTHON_ASSUME_SAFE_MACROS 0
   #undef CYTHON_UNPACK_METHODS
@@ -183,8 +185,9 @@
   #define CYTHON_FAST_PYCALL 0
   #undef CYTHON_PEP489_MULTI_PHASE_INIT
   #define CYTHON_PEP489_MULTI_PHASE_INIT 0
-  #undef CYTHON_USE_TP_FINALIZE
-  #define CYTHON_USE_TP_FINALIZE 1
+  #ifndef CYTHON_USE_TP_FINALIZE
+    #define CYTHON_USE_TP_FINALIZE 1
+  #endif
   #undef CYTHON_USE_DICT_VERSIONS
   #define CYTHON_USE_DICT_VERSIONS 0
   #undef CYTHON_USE_EXC_INFO_STACK