Allow build.zig scripts to define module definition files (.def) when building win32 dlls.

lib/std/Build.zig

@@ -832,6 +832,8 @@ pub const LibraryOptions = struct {
     /// Can be set regardless of target. The `.manifest` file will be ignored
     /// if the target object format does not support embedded manifests.
     win32_manifest: ?LazyPath = null,
+    /// Win32 module definition file (.def).
+    win32_module_definition: ?LazyPath = null,
 };
 
 pub fn addLibrary(b: *Build, options: LibraryOptions) *Step.Compile {
@@ -846,6 +848,7 @@ pub fn addLibrary(b: *Build, options: LibraryOptions) *Step.Compile {
         .use_lld = options.use_lld,
         .zig_lib_dir = options.zig_lib_dir,
         .win32_manifest = options.win32_manifest,
+        .win32_module_definition = options.win32_module_definition,
     });
 }