icu: only patch-out xlocale if using glibc

pkgs/development/libraries/icu/base.nix

···
   '';
 
   # https://sourceware.org/glibc/wiki/Release/2.26#Removal_of_.27xlocale.h.27
-  postPatch = "substituteInPlace i18n/digitlst.cpp --replace '<xlocale.h>' '<locale.h>'";
+  postPatch = if stdenv ? glibc
+    then "substituteInPlace i18n/digitlst.cpp --replace '<xlocale.h>' '<locale.h>'"
+    else null; # won't find locale_t on darwin
 
   inherit patchFlags patches;