Warning on libc not found for non-existing compiler

[alalazo]

Steps to reproduce

Using a compilers.yaml with compiler definitions for two different systems, with the non-host definitions in a path that does not exist:

compilers:
- compiler:
    spec: gcc@=9.4.0
    paths:
      cc: /usr/bin/gcc
      cxx: /usr/bin/g++
      f77: /usr/bin/gfortran
      fc: /usr/bin/gfortran
    flags: {}
    operating_system: ubuntu20.04
    target: x86_64
    modules: []
    environment: {}
    extra_rpaths: []
- compiler:
    spec: gcc@=9.3.0
    paths:
      cc: /some/path/gcc
      cxx: /some/path/g++
      f77: /some/path/gfortran
      fc: /some/path/gfortran
    flags: {}
    operating_system: ubuntu22.04
    target: x86_64
    modules: []
    environment: {}
    extra_rpaths: []

Concretitization always warns about not finding libc for the non-existing compiler:

$ spack spec zlib-ng

Error message

==> Warning: cannot detect libc from gcc@=9.3.0. The compiler will not be used during concretization.
Input spec
--------------------------------
 -   zlib-ng

Concretized
--------------------------------
 -   zlib-ng@2.1.6%gcc@9.4.0+compat+new_strategies+opt+pic+shared build_system=autotools arch=linux-ubuntu20.04-icelake
 -       ^gcc-runtime@9.4.0%gcc@9.4.0 build_system=generic arch=linux-ubuntu20.04-icelake
[e]      ^glibc@2.31%gcc@9.4.0 build_system=autotools arch=linux-ubuntu20.04-icelake
 -       ^gmake@4.4.1%gcc@9.4.0~guile build_system=generic arch=linux-ubuntu20.04-icelake

Information on your system

• Spack: 0.23.0.dev0 (7f3dd38)
• Python: 3.11.5
• Platform: linux-ubuntu20.04-icelake
• Concretizer: clingo

@becker33

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output