在更新软件包（在 Miktex 上）后，几乎没有任何东西可以编译。例如：
\documentclass{article}
\usepackage{libertine}
\begin{document}
plop
\end{document}
导致以下错误：
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:10: Undefined control sequence. [\ifxetex]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:12: Extra \else. [\else]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:12: Undefined control sequence. [\else\ifluatex]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:14: Extra \else. [\else]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:16: Extra \fi. [\fi]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:16: Extra \fi. [\fi\fi]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:289: Undefined control sequence. [ \defaultfontfeatures]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:290: LaTeX Error: Missing \begin{document}. [ L]
C:\Users\TL\AppData\Roaming\MiKTeX\2.9\tex\latex\libertine\libertine.sty:296: Undefined control sequence. [ \setmainfont]
然而，该包国际纺织展览会已安装，并且没有伊夫克斯下载包。好像伊普泰克提供\ifXeTeX，而不是ifxetex（真的吗！？！......）。 也存在类似的问题\ifluatex。
如何修复这个问题？