Update BUILDING.md

[brendanpolin]

Edited the vcpkg instructions for windows so they just work if you copy and paste

Complete description of the bug fix or feature that this pull request implements

Checklist before submitting the pull request, to maximize the chances that the pull request will be accepted

• Read CONTRIBUTING.md, a link to which appears under "Helpful resources" below. That document discusses general guidelines for contributing to libjpeg-turbo, as well as the types of contributions that will not be accepted or are unlikely to be accepted.
• Search the existing issues and pull requests (both open and closed) to ensure that a similar request has not already been submitted and rejected.
• Discuss the proposed bug fix or feature in a GitHub issue, through direct e-mail with the project maintainer, or on the libjpeg-turbo-devel mailing list.