cmake: check if NIX_CC exists before using it

[KiruyaMomochi]

Description of changes

Due to the patch added by us, cmake package does not work when NIX_CC environment variable does not exists.
When $ENV{NIX_CC} is empty, the condition IS_DIRECTORY AND EXISTS /nix-support/orig-libc AND EXISTS /nix-support/orig-libc-dev is invalid.

nixpkgs/pkgs/development/tools/build-managers/cmake/001-search-path.diff

Lines 24 to 34 in e17b079

	# List common include file locations not under the common prefixes.
	+if(IS_DIRECTORY $ENV{NIX_CC}
	+ AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc
	+ AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc-dev)
	+ file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc" _nix_cmake_libc)
	+ file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc-dev" _nix_cmake_libc_dev)
	+else()
	+ set(_nix_cmake_libc @libc_lib@)
	+ set(_nix_cmake_libc_dev @libc_dev@)
	+endif()
	+

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[wentasah]

It seems this could fix the problem I've just encountered. When I call cmake manually, it fails with the following error:

CMake Error at /nix/store/366p8r6ja02pv27xrgpafha353rzvglv-cmake-cursesUI-qt5UI-3.24.1/share/cmake-3.24/Modules/Platform/UnixPaths.cmake:51 (if):
  if given arguments:

    "IS_DIRECTORY" "AND" "EXISTS" "/nix-support/orig-libc" "AND" "EXISTS" "/nix-support/orig-libc-dev"

  Unknown arguments specified
Call Stack (most recent call first):
  /nix/store/366p8r6ja02pv27xrgpafha353rzvglv-cmake-cursesUI-qt5UI-3.24.1/share/cmake-3.24/Modules/Platform/Linux.cmake:85 (include)
  /nix/store/366p8r6ja02pv27xrgpafha353rzvglv-cmake-cursesUI-qt5UI-3.24.1/share/cmake-3.24/Modules/CMakeSystemSpecificInformation.cmake:37 (include)

Things work when I set NIX_CC manually.

I'm not testing this PR due to high number of rebuilds :-(.

[wentasah]

I managed to test this PR. I built just cmake and verified that manual cmake invocation does not fail with my projects.

[trofi]

The fix looks good. Thank you!

[LucioFranco]

Thanks for the patch and the reviews! I can confirm this fixes my issue on my M1 Mac! Thanks!

Would it make sense to backport this to 22.05?

[stephank]

The run-time libc detection was added after 22.05 in #181431. Here's the simpler patch currently in 22.05, which doesn't have the new checks: https://github.com/NixOS/nixpkgs/blob/nixos-22.05/pkgs/development/tools/build-managers/cmake/search-path.patch

I don't think #181431 should be backported, so this shouldn't be either.

Though, should we be targeting staging for this, considering the Darwin rebuilds?

[lovesegfault]

Yup, this needs to target staging