Introducing some "global" build out-of-source variable

[Ryan Schmidt]

On Mar 23, 2018, at 15:10, Mojca Miklavec wrote:

> In the absence of any PortGroup implementing the build system, this
> should make sure that autotools-based ports would properly build out
> of source without changing anything else.

I didn't think that autotools in general supported out-of-source builds. Certainly individual projects might support it, but I got the impression that this was much more of an opt-in situation with autotools than it is with cmake where almost all cmake-using projects work with out-of-source builds.

I guess we could add build.out_of_source to base, set it up in base so that it works for those autotools projects that support out-of-source builds, and leave it up to ports to enable it if desired, and for build system portgroups to override with behavior that works for them.

This all presumes that out-of-source builds are useful. I'm never sure if they are. Some people sure seem to like them, but I thought their primary use was when you want to cross-compile for different systems, each to their own build directory, or when you are doing development in a long-running git clone of the source and you want to keep it clean of build artifacts. MacPorts is building only for one system, and always starts from a clean build directory because it always starts by extracting the source, so these benefits don't seem to apply to MacPorts.