texi2pdf: environment variables with spaces don't work

[Werner LEMBERG]

For testing purposes I generated a 'luatex-plain' format with

```
mtxrun --generate
mtxrun --script plain --make
```

However, using it as

```
PDFTEX="luatex --fmt=luatex-plain" texi2pdf foo.texi
```

fails with

```
texi2dvi: TeX neither supports -recorder nor outputs \openout lines in its log 
file
```

Running the `openout.tex` test manually, i.e., calling

```
luatex --fmt=luatex-plain --file-line-error </dev/null '\input' ./openout.tex
```

works just fine.

It seems that the `PDFTEX` environment variable doesn't accept
commands with arguments.


    Werner