ExternalProject: Add support for using source mirrors
When building a project I would like to be able to specify a list of mirrors for fetching external source. This should work for all protocol types and should be configure-able outside of the project cmakelists, so a user can customize the mirrors without changing the project files.
Yocto does this well with "SOURCE_MIRRORS" and is a good example https://docs.yoctoproject.org/dev-manual/efficiently-fetching-sources.html?highlight=mirrors
Cmake external project already provides a very simple mechanism but it has the following issues
- it requires the cmake files to change to customize the mirrors
- you cannot list both local paths and remote mirrors in the mirror list
Edited by Adam Boseley