diff --git a/build.zig b/build.zig
index 887e977..94f09cd 100644
--- a/build.zig
+++ b/build.zig
@@ -18,7 +18,7 @@ pub fn build(b: *std.Build) !void {
     minisign.addIncludePath(b.path("src"));
     minisign.addSystemIncludePath(.{ .cwd_relative = "/opt/homebrew/include" });
     minisign.addSystemIncludePath(.{ .cwd_relative = "/usr/local/include" });
-    minisign.defineCMacro("_GNU_SOURCE", "1");
+    minisign.root_module.addCMacro("_GNU_SOURCE", "1");
     const source_files = &.{ "src/base64.c", "src/get_line.c", "src/helpers.c", "src/minisign.c" };
     minisign.addCSourceFiles(.{ .files = source_files });