Rework --data's handling of sourceFiles and static library dependencies.

Split --data=libs into two: --data=libs (for installed/system libs) and --data=lib-files (for dependency libs) ...

This is needed because compilers/linkers often expect the two types of libs to be passed in differently: System/installed libs (--data=libs) must be provided like "-Lname" which the linker automatically converts to "path/lib*.a". Non-installed libs, such as dub dependencies, are passed with full filepath just like an object file. Your linker may vary.

Add "targets" field to "dub describe" output.

Change Build(Options/Requirements) to BitFlags.

Avoid adding duplicate string imports in BuildSettings.addStringImportFiles. ...

Prepares for fixing #331.

Fixed issue where static libraries would be generated instead of object files in singleFile build mode ...

+ Added object TargetType that is used during singleFile build mode

Put BuildSettings into its own module and invoke DMD to determine the actual build platform. ...

Currently only for DMD, the build platform is now determined by actually invoking the compiler with the proper -mX switches instead of guessing from the compiler run that was used to build DUB.