document raison d'être for min_runtime_version

recipe/meta.yaml

@@ -1,5 +1,12 @@
 {% set build_num = 3 %}
-{% set min_runtime_version = 13 %}
+# generally, the runtime version of libstdcxx needs to be at least as high
+# as the compiler; however, wherever libstdcxx changes the default ABI version
+# of a symbol, we need to ensure at least that version at runtime also for
+# packages built with older compilers (because that build environment will
+# pull in the newest libstdcxx, and then depend on that new symbol version).
+# C.f. https://johannst.github.io/notes/development/symbolver.html resp.
+# https://github.com/search?q=repo%3Agcc-mirror%2Fgcc%20_GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE&type=code
+{% set min_runtime_version = 13 if ctng_gcc.split('.')[0] | int >= 13 else 12 %}
 
 package:
   name: ctng-compiler-activation