treewide: move NIX_CFLAGS_COMPILE to the env attrset
`nix-eval-jobs --flake ".#legacyPackages.x86_64-linux" --workers 3 2>/dev/null --impure | jq 'select(.error?) | select(.error | match ("attribute set can only contain derivation") )'`
and
`nix-eval-jobs --flake ".#legacyPackages.aarch64-darwin" --workers 3 2>/dev/null --impure | jq 'select(.error?) | select(.error | match ("attribute set can only contain derivation") )'`
no longer has output so this could be all of the remaining issues
with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
Apparently on x86_64-darwin there are also C files compiled,
and that doesn't go well with -std=c++*
https://hydra.nixos.org/build/174111870
Fortunately the flag only seems needed with gcc 11 so far.
continuation of #109595
pkgconfig was aliased in 2018, however, it remained in
all-packages.nix due to its wide usage. This cleans
up the remaining references to pkgs.pkgsconfig and
moves the entry to aliases.nix.
python3Packages.pkgconfig remained unchanged because
it's the canonical name of the upstream package
on pypi.
