Update Build Settings.

Xcode 5.1.1 suggested enforcing stricter build settings. Update the xcconfig files to reflect the build environment currently available in Xcode.
	project/Mac OS X/xcconfig/compiler__common.xcconfig
	project/Mac OS X/xcconfig/compiler__debug.xcconfig
	project/Mac OS X/xcconfig/compiler__release.xcconfig

Add virtual destructors to classes with virtual member functions.
	src/Compiler/AST/Context.h
	src/Compiler/AST/Context.cpp
	src/Compiler/AST/Expression.h
	src/Compiler/AST/Expression.cpp
	src/Compiler/AST/Statement.h
	src/Compiler/AST/Statement.cpp
	src/Compiler/CodeGen/IPlatform.h
	src/Compiler/Diagnostics/IReporting.h