conf: discover bundled configuration beside the loaded shared library

[GitHub pull_request - opened <github@alsa-project.org>]

alsa-project/alsa-lib pull request #540 was opened from Robolightning:

When a shared ALSA library is distributed with its original configuration tree, the compiled build-machine data directory may be unavailable after relocation. Resolve a readable `alsa-config/alsa.conf` beside the actually loaded DSO before falling back to the compiled directory. An absolute explicit `ALSA_CONFIG_DIR` keeps precedence.

Cache an owned path with `pthread_once`. Resolve the DSO from a static local function address so another globally loaded libasound cannot substitute the lookup target. This does not mutate process environment variables or change PCM plugins, exported symbols, or compiled-directory fallback.

Validated on the v1.2.14 source baseline with the original `make check` (configuration and MIDI tests), all 1,835 exported SDK symbols unchanged, and real null/plug:null PCM I/O: four threads, 32,768 frames per case. Separate native processes checked relocated default, explicit user directory, compiled fallback, and explicit-handle use alongside an older globally loaded library. The private producer used the existing `--enable-symbolic-functions` option; no data-symbol binding change is proposed.

The scope is SDK configuration discovery. An unrelated Python extension's external calls can still resolve to an older library explicitly preloaded RTLD_GLOBAL; this change makes no claim to isolate those calls. Physical sound hardware was unavailable.

Request URL   : https://github.com/alsa-project/alsa-lib/pull/540
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/540.patch
Repository URL: https://github.com/alsa-project/alsa-lib