writing LaTeXML's TeX packages should be optional

For my distributed LaTeXML processing of arXiv, I want to automate the LaTeXML installation on the various machines we are using via a simple cpanminus invocation:
```
cpanm git://github.com/brucemiller/LaTeXML.git@commitSHA
```

When I tried that, it turned out there was a single failure - LaTeXML always attempts to write its TeX package files when a TeX installation is found, assuming it has the required permissions. I have added a check that throws a warning and skips that part of the installation when the texmf directory isn't writable. If I am not mistaken the TeX files are only needed for the manual generation in the first place?