Add functionality to build Windows shared lib dll

[mysticmind]

• Add missing implementation for build_shared target in Makefile.mvcc
• Windows shared lib dll build requires a module-definition file pg_query.def to export the required functions. Add functionality to create the file dynamically by extracting the info from pg_query.h using a Powershell script. It is wired via create_def target in Makefile.mvcc

PS: I am in the process of building a .NET based lib for libpg_query and wanted cross-platform shared libs of libpg_query targeting *nix, OSX and Windows. Noticed the DLL build for Windows was missing hence this PR.