freeglut: set correct library name (#39300)

var/spack/repos/builtin/packages/freeglut/package.py

@@ -71,3 +71,7 @@ def cmake_args(self):
         ]
 
         return args
+
+    @property
+    def libs(self):
+        return find_libraries(["libglut"], root=self.prefix, recursive=True)