Change to the treatment of BeastConfig.h

* No longer requires its own compiler include path
* Includes use relative paths to locate the file
* Client applications include the file themselves
* Inclusion of BeastConfig.h can be controlled via preprocessor directive

beast/net/Net.cpp

@@ -17,7 +17,9 @@
 */
 //==============================================================================
 
-#include "BeastConfig.h"
+#if BEAST_INCLUDE_BEASTCONFIG
+#include "../../BeastConfig.h"
+#endif
 
 #include "../Config.h"