We can actually specify an old OS version perfectly well in the universal cross file, and it'll work for both slices (the ARM one will just get 11 instead). So do that, and document it. Also print out the target in the config summary.

meson.build

@@ -327,31 +327,43 @@ if system == 'darwin'
     '-exported_symbols_list', meson.source_root() / 'vamp/vamp-plugin.list'
   ]
 
-  have_version_min = false
-  foreach arg: get_option('cpp_args')
-    if arg.contains('version-min')
-      have_version_min = true
-    endif
-  endforeach
-
   if architecture == 'aarch64'
     arch_flags += [
       '-arch', 'arm64',
     ]
-    if not have_version_min
-      arch_flags += [ '-mmacosx-version-min=11' ]
-    endif
   elif architecture == 'x86_64'
     arch_flags += [
       '-arch', 'x86_64',
     ]
-    if not have_version_min
-      arch_flags += [ '-mmacosx-version-min=10.13' ]
-    endif
   else # begin architecture != 'aarch64' or 'x86_64'
     error('Build for architecture ' + architecture + ' is not supported on this platform')
   endif # end architecture
 
+  have_version_min = false
+  foreach arg: get_option('cpp_args')
+    if arg.contains('version-min')
+      have_version_min = true
+      bits = arg.split('=')
+      if bits.length() > 1
+	config_summary += { 'Target OS': bits[1] + '+' }
+      else
+	config_summary += { 'Target OS': '(unknown)' }
+      endif
+    endif
+  endforeach
+
+  if not have_version_min
+    message('Using default minimum target OS version')
+    message('(consider specifying this in cross-file if earlier target is desired)')
+    if architecture == 'aarch64'
+      arch_flags += [ '-mmacosx-version-min=11' ]
+      config_summary += { 'Target OS': '11+' }
+    else 
+      arch_flags += [ '-mmacosx-version-min=10.13' ]
+      config_summary += { 'Target OS': '10.13+' }
+    endif
+  endif
+
 elif system == 'windows'
   feature_defines += ['-D_WIN32', '-DNOMINMAX', '-D_USE_MATH_DEFINES', '-DGETOPT_API=']
   if cpp.get_id() == 'msvc'