Backport Darwin TBD Fixes to 20.09

[TravisWhitaker]

The tip of nixos-20.09 isn't usable on MacOS Big Sur because it's missing the new TBD-oriented stdenv:

$ nix-build -A hello --check
checking outputs of '/nix/store/19nkhsryfvzp66fkl1mf5026xj7gxdqm-hello-2.10.drv'...
unpacking sources
unpacking source archive /nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz
source root is hello-2.10
setting SOURCE_DATE_EPOCH to timestamp 1416139241 of file hello-2.10/ChangeLog
patching sources
configuring
configure flags: --disable-dependency-tracking --prefix=/nix/store/180066650sp74bhy3fxl4234z8vdnhnz-hello-2.10
checking for a BSD-compatible install... /nix/store/l5y1dp8cbsjysvzvivrac5jrk2bpdh4s-coreutils-8.31/bin/install -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /nix/store/l5y1dp8cbsjysvzvivrac5jrk2bpdh4s-coreutils-8.31/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking for gcc... clang
checking whether the C compiler works... no
configure: error: in `/private/tmp/nix-build-hello-2.10.drv-0/hello-2.10':
configure: error: C compiler cannot create executables
See `config.log' for more details
builder for '/nix/store/19nkhsryfvzp66fkl1mf5026xj7gxdqm-hello-2.10.drv' failed with exit code 77
error: build of '/nix/store/19nkhsryfvzp66fkl1mf5026xj7gxdqm-hello-2.10.drv' failed

Is there any interest in backporting the TBD stdenv to the 20.09 branch?

[TravisWhitaker]

@thefloweringash @LnL7 @matthewbauer do you have any thoughts on how tough this would be to accomplish? Does this PR (#107013) represents the current good state of the Darwin stdenv?

[Gabriella439]

My understanding is that the key pull request that needs to be backported is this one:

#98541

However, that pull request cannot be easily backported to 20.09 because it depends on a few other changes that were made post-20.09

[Gabriella439]

Actually, this appears to be easier than I thought. I have a PR to backport the Big Sur fixes to 20.09 here: #121040