plink: Patch to fix build, add test

plink contains bad C that old versions of gcc used to accept. clang and gcc-4.8 both reject unpatched plink. Debian applies a patch to fix the code. This pull request applies the same patch. With the patch, the `fails_with :clang` stanza is no longer necessary.

The test is also improved to run against example data files that were already being installed.