shared library, so
        assume that *all* Unix C compilers do,
        ignoring even GCC's "-static" option.

        >>> compiler = UnixCCompiler()
        >>> compiler._library_root = lambda dir: dir
        >>> monkeypatch = getfixture('monkeypatch')
        >>> monkeypatch.setattr(os.path, 'exists', lambda d: 'existing' in d)
        >>> dirs = ('/foo/bar/missing', '/foo/bar/existing')
        >>> compiler.find_library_file(dirs, 'abc').replace('\\', '/')
        '/foo/bar/existing/libabc.dylib'
        >>> compiler.find_library_file(reversed(dirs), 'abc').replace('\\', '/')
        '/foo/bar/existing/libabc.dylib'
        >>> monkeypatch.setattr(os.path, 'exists',
        ...     lambda d: 'existing' in d and '.a' in d)
        >>> compiler.find_library_file(dirs, 'abc').replace('\\', '/')
        '/foo/bar/existing/libabc.a'
        >>> compiler.find_library_file(reversed(dirs), 'abc').replace('\\', '/')
        '/foo/bar/existing/libabc.a'
        c