link to system provided iconv by default on macOS

It's also possible to avoid the complexity in the documentation by
disabling iconv support by default but I would like to keep the same
defaults as upstream. I would like to avoid injecting unnecessary
opinionated choices into this port.

Author: Techatrix

README.md

@@ -20,6 +20,22 @@ You can then import `libxml2` in your `build.zig` with:
 const libxml2_dependency = b.dependency("libxml2", .{
     .target = target,
     .optimize = optimize,
+
+    // libxml2 will try to link to iconv on macOS by default which will
+    // fail when cross-compiling. You may disable iconv support for or
+    // link against GNU libiconv which is licensed under LGPL.
+
+    // disable iconv support on all targets
+    // .iconv = false,
+
+    // disable iconv support when compiling to macOS
+    // .iconv = !target.result.os.tag.isDarwin(),
+
+    // disable iconv support when cross-compiling to macOS
+    // .iconv = !(target.query.isNativeOs() and target.result.os.tag.isDarwin()),
+
+    // Use GNU libiconv on macOS which is licensed under LGPL.
+    // .@"iconv-impl" = if (target.result.os.tag.isDarwin()) else null,
 });
 your_exe.linkLibrary(libxml2_dependency.artifact("xml"));
 ```

build.zig

@@ -187,7 +187,7 @@ pub fn build(b: *std.Build) void {
     if (http and target.result.os.tag == .windows) xml_lib.root_module.linkSystemLibrary("ws2_32", .{});
 
     if (iconv) {
-        if (b.systemIntegrationOption("iconv", .{})) {
+        if (b.systemIntegrationOption("iconv", .{ .default = target.result.os.tag.isDarwin() })) {
             xml_lib.root_module.linkSystemLibrary("iconv", .{});
         } else {
             const IconvImpl = enum { libc, libiconv, win_iconv };