msvc: allow dlls without lib file

On Windows it is possible to create a dll consisting only of a resource (message) file which is used for the eventlog. Because it doesn't export any function the msvc linker doesn't create a lib file. Due to that boost build always thinks that the dll needs to be build. i have registered the generator twice with the feature suppress-import-lib (there is no use/doc of it) and only generate the IMPORT_LIB target for one. i haven't figured out another way to achieve that.
What is missing that /NOENTRY needs to be passed to the linker. The pull request is to gain ideas/comments how to fully implement the feature for dlls without any exports.
regards,
bernhard