cannot build on NixOS due to undefined C functions in `ext_s4u`

[lp-magenta]

On NixOS installing gssapi fails with some GCC errors "implicit declaration of function ‘gss_acquire_cred_impersonate_name’" and "implicit declaration of function ‘gss_add_cred_impersonate_name’" when compiling ext_s4u.c:

      gcc -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall -fPIC -DPy_LIMITED_API=51052544 -Igssapi/raw -I/home/lp/.cache/pypoetry/virtualenvs/dipex-GkntI14x-py3.13/include -I/nix/store/qwb5ygz9k8gs5ql9bpxbrsrv12r1icgm-python3-3.13.12/include/python3.13 -c gssapi/raw/ext_s4u.c -o build/temp.linux-x86_64-cpython-313/gssapi/raw/ext_s4u.o -I/nix/store/9jsgjswa0jmgbwk3l8ha2dy45yr4dz62-krb5-1.22.1-dev/include
      gssapi/raw/ext_s4u.c: In function ‘__pyx_pf_6gssapi_3raw_7ext_s4u_acquire_cred_impersonate_name’:
      gssapi/raw/ext_s4u.c:3282:28: error: implicit declaration of function ‘gss_acquire_cred_impersonate_name’ [-Wimplicit-function-declaration]
       3282 |         __pyx_v_maj_stat = gss_acquire_cred_impersonate_name((&__pyx_v_min_stat), __pyx_v_impersonator_cred->raw_creds, __pyx_v_name->raw_name, __pyx_v_input_ttl, __pyx_v_desired_mechs, __pyx_v_c_usage, (&__pyx_v_creds), (&__pyx_v_actual_mechs), (&__pyx_v_actual_ttl));
            |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      gssapi/raw/ext_s4u.c: In function ‘__pyx_pf_6gssapi_3raw_7ext_s4u_2add_cred_impersonate_name’:
      gssapi/raw/ext_s4u.c:3945:28: error: implicit declaration of function ‘gss_add_cred_impersonate_name’ [-Wimplicit-function-declaration]
       3945 |         __pyx_v_maj_stat = gss_add_cred_impersonate_name((&__pyx_v_min_stat), __pyx_v_raw_input_cred, __pyx_v_impersonator_cred->raw_creds, __pyx_v_name->raw_name, (&__pyx_v_mech->raw_oid), __pyx_v_c_usage, __pyx_v_input_initiator_ttl, __pyx_v_input_acceptor_ttl, (&__pyx_v_creds), (&__pyx_v_actual_mechs), (&__pyx_v_actual_initiator_ttl), (&__pyx_v_actual_acceptor_ttl));
            |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      error: command '/run/current-system/sw/bin/gcc' failed with exit code 1

I have installed the gcc, krb5 and krb5.dev nix packages which should have all the necessary dependencies to build gssapi.

Steps to reproduce

• Run NixOS 25.11
• Add gcc, krb5 and krb5.dev to your system packages
• Enable dynamically linked libraries (needed to run Python): programs.nix-ld.enable = true;
• Rebuild
• Create and enter a virtual environment
• Run pip install gssapi

[jborean93]

I'm afraid I've never really used NixOS so I don't really know what might be going on here and probably won't have time to investigate anytime soon. There doesn't seem to be anything special about these 2 functions and how they are defined compared to other extension functions.

Probably the only thing I would check is to ensure that the Cython version being used to build the .c files here matches the build requirements. I don't know whether NixOS disables build isolation in general or whether that is still being used by pip here.