libmng: fix trying to run configure on an already configured directory fixes #2959 (#2961)

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

@@ -42,3 +42,7 @@ def patch(self):
         # jpeg requires stdio to beincluded before its headrs.
         filter_file(r'^(\#include \<jpeglib\.h\>)',
                     '#include<stdio.h>\n\\1', 'libmng_types.h')
+
+    @run_before('configure')
+    def clean_configure_directory(self):
+        make('distclean')