Comment 8 for bug 414431

Using synctex can either be decided at compile time using the flag "-synctex=1" when calling pdflatex, or in the .tex file itself with the line "\synctex=1" in the preamble. Either option suffices to generate synctex data, it is not needed to have both.

So in general it is impossible to detect from the .tex file only whether the user wants to use synctex or not, and when the information is present in the .tex file (i.e. the line "\synctex=1" is present), it is actually useless for rubber since synctex data is then automatically generated by pdflatex anyway.

Hence it seems that it would be best to have a "-synctex=1" flag for rubber that would just get transferred directly to pdflatex.

NB: to be more precise about the syntax, "pdflatex --help" yields:
    [...]
    -synctex=NUMBER generate SyncTeX data for previewers if nonzero
    [...]