The scons build system is python-based and has a binary named scons. Unlike CMake, it cannot generate makefiles so we end up having to override the build, install, and check phases. I have added the setupHook to the scons package so that integration requires no unique steps - just putting scons in nativeBuildInputs should be enough. sconsFlags controls the flags specifically passed to scons while buildFlags, installFlags, and checkFlags should still be usable. Some packages use different names for the prefix flag. In those cases you will have to set "prefixKey" to something like "PREFIX=" as there are multiple names for the "prefix" used in scons.
The waf build system is python-based and hosted locally in each package in the executable file named "waf". Unlike CMake, it cannot generate makefiles so we end up having to override the configure, build, and install phases. I've tried to keep these as close to what's in setup.sh as possible. If there is no waf file in the root directory, then we just copy the one hosted in Nixpkgs. Otherwise the only thing you have to add to a package using Waf is "wafHook" into nativeBuildInputs. wafFlags controls the flags specifically passed to waf while configureFlags, buildFlags, and installFlags are still used as in the generic builder.
This will make the list much easier to re-use, eg. for `nixosTests`
The drawback is that this approaches makes the
```
nix-build release.nix -A tests.opensmtpd.x86_64-linux
```
command about twice as slow (3s to 6s): it now has to evaluate `nixpkgs`
once for each architecture, instead of just having the hardcoded list of
tests that allowed to say “ok just evaluate for x86_64-linux”.
On the other hand, complete evaluation of `release.nix` should be much
faster because we no longer import `nixpkgs` for each test: testing with
the following command went from 30s to 18s, and that's just for a few
tests.
```
time nix-instantiate --eval --strict nixos/release.nix -A tests.nat
```
I initially wanted to test on the whole `release.nix`, but there are too
many broken tests and it takes too long to eval them all, especially
compared to the fact that the current implementation breaks some setup.
Given developers can just `nix-build nixos/tests/my-test.nix`, it sounds
like an overall win.
