ldns: fix cross compiling on darwin

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

libs/ldns/patches/001-fix-cross-compile-on-darwin.patch

@@ -0,0 +1,11 @@
+--- a/configure.ac
++++ b/configure.ac
+@@ -842,7 +842,7 @@ ACX_CHECK_FORMAT_ATTRIBUTE
+ ACX_CHECK_UNUSED_ATTRIBUTE
+ 
+ # check OSX deployment target, if needed
+-if echo $build_os | grep darwin > /dev/null; then
++if echo $target_os | grep darwin > /dev/null; then
+   sdk_p=`xcode-select -print-path`;
+   sdk_v="$( /usr/bin/xcrun --show-sdk-version )";
+   case $sdk_v in