cmake and cmake-ninja should default to builddir=true

[aleixpol]

If a project needs to be built in-place it's a bug in the project then the maintainer can disable it, but by default the right thing to do is to build elsewhere.

[alexlarsson]

It felt like a bunch of cmake projects didn't support builddir=true at some point when i was testing things, so i fell back to the "most safe" approach. Switching at this point is a minor api change though, as modules not supporting it may break.

[aleixpol]

Well, if it's possible to have it changed, I'd welcome it. If the cosmos may break loose, then we don't and that's fine too.
Or we change it in a year hoping people have learned to cmake by then.

[TingPing]

Is there any actual downside to builddir=false with cmake? The normal problems don't really apply since everything is thrown away between changed builds anyway.

[aleixpol]

Having CMAKE_SOURCE_DIR equal to CMAKE_BINARY_DIR can lead to the scripts overwriting source files unintentionally because the developer didn't think it was a use-case to not use a separate build directory (have seen it happen).

[bbhtt]

I think this should apply to meson too. fdsdk uses this as the default too.

We can probably switch the default for 1.5.0