Update links in building.md to use https rather than http

Building.md file contains a number of HTTP links that would need to be converted to HTTPS in order for clicking on the links on the OpenJDK web site at https://openjdk.org/groups/build/doc/building.html to work.

For example, the Cygwin link in "A functioning Cygwin environment is required for building the JDK on Windows." is not functional on the web site.

Clicking on it results in error messages in a browser console like

"Mixed Content: The page at 'https://openjdk.org/groups/build/doc/building.html' was loaded over HTTPS, but requested an insecure resource 'http://www.cygwin.com/'. This request has been blocked; the content must be served over HTTPS."