Fix MSVC compile errors.

Exported symbols have different symbol names
and msvc can't find it without VARCONF_API.
Allow to modify VARCONF_API with global flags.

For example when I want to build it as a static lib in msvc,
I don't need dllimport/dllexport, which is needed for dynamic libs.
I just add -D "VARCONF_API=" to the compiler.
Merge pull request #3 from sajty/master

Fix VARCONF_API for Visual Studio.