liblqr1: fix build with gcc15

• add "-std=gnu17" to env.NIX_CFLAGS_COMPILE, most updates upstream
  were in 2017 or before.

Fixes build failure with gcc15:

lqr_energy.c:452:1: error: conflicting types for
'lqr_carver_generate_rcache_bright'; have 'gdouble *(LqrCarver *)'
{aka 'double *(struct _LqrCarver *)'}
  452 | lqr_carver_generate_rcache_bright(LqrCarver *r)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../lqr/lqr_energy.h:27,
                 from lqr_energy.c:32:
../lqr/lqr_energy_priv.h:59:10: note: previous declaration of
'lqr_carver_generate_rcache_bright' with type 'gdouble *(void)'
{aka 'double *(void)'}
   59 | gdouble *lqr_carver_generate_rcache_bright();
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lqr_energy.c:471:1: error: conflicting types for
'lqr_carver_generate_rcache_luma'; have 'gdouble *(LqrCarver *)'
{aka 'double *(struct _LqrCarver *)'}
  471 | lqr_carver_generate_rcache_luma(LqrCarver *r)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...

liblqr1: cleanup

• replace rec with finalAttrs
• remove with lib; from meta
• replace sha256 with hash
  nix hash to-sri --type sha256 "10mrl5k3l2hxjhz4w93n50xwywp6y890rw2vsjcgai8627x5f1df"
  sha256-rgVX+hEGRfWY1FvwDBLy5nLPOyh2JE4+lB0KOmahuYI=

Tested build with:

nix-build --expr 'with import ./. {}; liblqr1.override { stdenv = gcc15Stdenv; }'

Part of fixes for gcc15 update:
#440456

Upstream is not very active, most updates were before 2017 and
other distros also fixed gcc15 issues by adding -std=gnu17 or -std=gnu11:
https://gitlab.archlinux.org/archlinux/packaging/packages/liblqr/-/commit/5c2eb2a39389c874f2c019cad255b25db29a52c7
https://src.fedoraproject.org/rpms/liblqr-1/c/6f74801f26aee9c7030aca920813fcfeb4d856a4
https://build.opensuse.org/requests/1267258/changes

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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