xmlsec: fix build on macOS (#101319)

* xmlsec: fix build on macOS

xmlsec detects the Darwin platform and uses the -D flag with base64, but nix uses GNU base64 which requires -d.

This patch removes the platform test and always uses the -d flag.

pkgs/development/libraries/xmlsec/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation {
 
   patches = [
     ./lt_dladdsearchdir.patch
-  ];
+  ] ++ stdenv.lib.optionals stdenv.isDarwin [ ./remove_bsd_base64_decode_flag.patch ];
   postPatch = ''
     substituteAllInPlace src/dl.c
   '';

pkgs/development/libraries/xmlsec/remove_bsd_base64_decode_flag.patch

@@ -0,0 +1,12 @@
+--- a/tests/testEnc.sh	2020-04-20 14:30:32.000000000 -0400
++++ b/tests/testEnc.sh	2020-10-21 22:09:25.000000000 -0400
+@@ -405,9 +405,6 @@
+             else
+                 # generate binary file out of base64
+                 DECODE="-d"
+-                if [ "`uname`" = "Darwin" ]; then
+-		    DECODE="-D"
+-                fi
+                 cat "$topfolder/$base_test_name.data" | base64 $DECODE > $tmpfile.3
+                 execEncTest "$res_success" \
+                     "" \