[AGENT++] patch when using clang compiler

[Flaig Cyril]

Hello

I have built the library with g++. Since clang and gcc are binary compatible on linux, I also compiled our a tool that uses agentpp with clang version 3.2.

Clang does not support forward declaration of a template  with incomplete type: 
e.g.:
line 455 in thread.h:
#if !defined (AGENTPP_DECL_TEMPL_ARRAY_TASKMANAGER)
#define AGENTPP_DECL_TEMPL_ARRAY_TASKMANAGER
	AGENTPP_DECL_TEMPL template class AGENTPP_DECL Array<TaskManager>;
#endif

I have found in agent++.h that for MacOS the forward declaration are disabled. When they are also disabled for clang then it compiles fine.
See attached patch.