Bugfix libcxx detection when using CC/CXX

[AbrilRBS]

Changelog: Bugfix: Fix libcxx detection when using CC/CXX env vars.
Docs: conan-io/docs#3509

Closes: #15399

[memsharded]

Please also check broken tests

[memsharded]

@czoido note this is a breaking change for those that started to use the detect_api feature, but we thought that being it declared as unstable and recently released, we are on time to change it.

The other alteranative was to opt-in via detect_compiler(return_compiler_exe=True) or the like

[czoido]

I also think that the chances to break are not so high, but my concern is that if anyone is using this, the error is going to be difficult to track, because it will look like this:

ERROR: Traceback (most recent call last):
File "/Users/test/conan/conan/cli/cli.py", line 270, in main
    cli.run(args)
  File "/Users/test/conan/conan/cli/cli.py", line 180, in run
    command.run(self._conan_api, args[0][1:])
  File "/Users/test/conan/conan/cli/command.py", line 126, in run
    info = self._method(conan_api, parser, *args)
  File "/Users/test/conan/conan/cli/commands/install.py", line 55, in install
    profile_host, profile_build = conan_api.profiles.get_profiles_from_args(args)
  File "/Users/test/conan/conan/api/subapi/profiles.py", line 66, in get_profiles_from_args
    profile_host = self._get_profile(host_profiles, args.settings_host, args.options_host, args.conf_host,
  File "/Users/test/conan/conan/api/subapi/profiles.py", line 88, in _get_profile
    profile = loader.from_cli_args(profiles, settings, options, conf, cwd)
  File "/Users/test/conan/conans/client/profile_loader.py", line 95, in from_cli_args
    tmp = self.load_profile(p, cwd)
  File "/Users/test/conan/conans/client/profile_loader.py", line 105, in load_profile
    profile = self._load_profile(profile_name, cwd)
  File "/Users/test/conan/conans/client/profile_loader.py", line 130, in _load_profile
    text = rtemplate.render(context)
  File "/Users/test/conan-virtual-env/lib/python3.9/site-packages/jinja2/environment.py", line 1301, in render
    self.environment.handle_exception()
  File "/Users/test/conan-virtual-env/lib/python3.9/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "<template>", line 2, in top-level template code
ValueError: too many values to unpack (expected 2)

ERROR: too many values to unpack (expected 2)

If we don't have a way to raise an error that guides users on how to fix the problem, I personally would prefer a non-breaking change, like adding a new function and deprecate the current one adding a deprecation warning.

[memsharded]

I think I agree, we can do it non-breaking and deprecating old behavior.