Bug 1245860 - Add --with-nspr-prefix to the list of old configure options. r=ted

build/moz.configure/old.configure

@@ -329,6 +329,7 @@ def old_configure_options(*options):
     '--with-macos-private-frameworks',
     '--with-macos-sdk',
     '--with-mozilla-api-keyfile',
+    '--with-nspr-prefix',
     '--with-nspr-cflags',
     '--with-nspr-libs',
     '--with-pthreads',