gn: Add install target Define an install_target GN template which installs include directories and a predefined list of libraries to a given prefix. The template is taking advantage of an install_target.py script which generates package config files for every named target. Bug: angleproject:8110 Change-Id: I6deb40fe9c54f11da7caf4fb5df59d3ce522df29 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5553797 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com> Auto-Submit: Antonio Caggiano <quic_acaggian@quicinc.com>