_build.sh: add Linux GLIBC TODO

This problem did not improve much in the last 20 years.

The `patchelf` trick as v0.18.0 (2023-04) is broken due to:
NixOS/patchelf#284
NixOS/patchelf#374
NixOS/patchelf#394

Links:
https://github.com/phusion/holy-build-box#problem-introduction
https://mesonbuild.com/Creating-Linux-binaries.html
https://andrewkelley.me/post/zig-cc-powerful-drop-in-replacement-gcc-clang.html

_build.sh

@@ -76,6 +76,7 @@ set -o xtrace -o errexit -o nounset; [ -n "${BASH:-}${ZSH_NAME:-}" ] && set -o p
 # TODO:
 #   - Change default TLS to BoringSSL (with OPENSSL_SMALL?) or LibreSSL?
 #   - linux: use musl.
+#   - linux: fix/mitigate missing GLIBC versioned symbols on systems older than the build machine
 #   - mac: Test -DSHARE_LIB_OBJECT=ON with curl 8.3.0.
 #   - Rename _BRANCH to CW_CONFIG internally.
 #   - Replace .zip with .tar.gz for all-packages artifact (in _ul.sh)?