strip out build tools like cmake, ninja, meson after bootstrapping Prefix for compat layer

[boegel]

We currently have several packages for build tools installed in the compat layer that we don't really want/need there, including dev-util/cmake, dev-util/meson, dev-util/ninja (possibly others too).

EasyBuild will install these tools as build dependencies in the software layer when they are needed, because the specific version being used may matter (I recall several cases where an older CMake version had to be used because of bugs/changes/regressions in more recent versions, for example).

I took a quick look at emerge --depclean for this, but it seems like build tools wouldn't be removed with this (see output produced by emerge --depclean --pretend).

@amadio: Any idea if there's an easy way to strip out all packages that were only installed because they are a build dependency for other packages?

[amadio]

As with linux-headers the easy way to check what's holding off removal is to pretend to remove the package with something like emerge -pvc cmake, for example. It could be that some package erroneously lists CMake or other build tool as runtime dependency, in which case you should file a bug, but Gentoo being a source distro, some of these tools are probably part of the system set. Just be careful with what you remove, because you can make it quite hard to update the system later if you remove things from system.