Node poll_instance callback is optional, check if it exists before executing.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55382 954f8c5b-7b00-dc11-b283-0030488c597c
Use extern "C" when including BKE_node.h in C++ compositor code, to avoid linker errors on windows (undefined NODE_INSTANCE_KEY_BASE).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55383 954f8c5b-7b00-dc11-b283-0030488c597c