Make pthread a dependency only on unix systems.

rockspecs/lua-llthreads-scm-0.rockspec

@@ -13,10 +13,18 @@ dependencies = {
 }
 build = {
 	type = "builtin",
+	platforms = {
+		unix = {
 			modules = {
 				llthreads = {
-			sources = {"src/pre_generated-llthreads.nobj.c"},
 					libraries = {"pthread"},
 				}
 			}
 		}
+	},
+	modules = {
+		llthreads = {
+			sources = {"src/pre_generated-llthreads.nobj.c"},
+		}
+	}
+}