conditional file parameters in make command?

[Erik]

I need to generate a PDF-file from subdocuments. I have this Makefile:
whole.pdf: 1.pdf 2.pdf 3.pdf 4.pdf 5.pdf 6.pdf 7.pdf 8.pdf 9.pdf
   gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=$@ -dBATCH $^

The problem is that a subdocument may begin on the backside of the last
page of the previous subdocument. This is bad if the printed document
should be divided into subdocuments. Therefore I need to make sure that
every subdocument begins on a new paper. A way to do this would be to
insert an empty 1-page document (empty.pdf) after each document that has
an odd number of pages:
whole.pdf: 1.pdf 2.pdf 3.pdf empty.pdf 4.pdf 5.pdf 6.pdf 7.pdf empty.pdf
8.pdf empty.pdf 9.pdf
   gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=$@ -dBATCH $^

That is certainly not the right way to do it. It will fail if I change
any subdocument so that its page count modulo 2 is changed.

By using pdfinfo (in the poppler package) I can find out whether a
particular PDF-file has an odd number of pages:
   echo $(pdfinfo file.pdf|egrep "^Pages: *[[:digit:]]+$"|sed "address@hidden:
*\([[:digit:]]\+\)address@hidden@")%2|bc

But which is the right way to build that into the Makefile?