Running doxygen tests with variable with spaces (#7819)

When having a run command like:
```
nmake tests TEST_FLAGS="--xhtml --keep --cfg GENERATE_HTMLHELP=YES --cfg HHC_LOCATION=\"c:\Program Files (x86)\HTML Help Workshop\hhc.exe\" --cfg HTML_FILE_EXTENSION=.html --cfg SEARCHENGINE=NO --id=1"
```
we get an error like
```
Not a doxygen configuration item, missing '=' sign: 'Files'.
```

this is due to the usage of the `split()`  that does a brute force split on spaces.
Making the splitting a bit more intelligent:
- splitting on `--`
- splitting on space after "command"

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