Build: limit Py_ENABLE_SHARED to modules using Python headers

And remove Python flags from nodes, no longer needed.

source/blender/python/CMakeLists.txt

@@ -1,5 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+if(WITH_PYTHON_MODULE)
+  add_definitions(-DPy_ENABLE_SHARED)
+endif()
+
 add_subdirectory(intern)
 add_subdirectory(generic)
 add_subdirectory(gpu)