man: make error message clearer for missing xstlproc

man: make error message clearer for missing xstlproc

We can't do at configure phase since we actually ship the built man pages with
dist, so it's fine not having xsltproc if building from dist. If building from
the repository, it's better to have have a better message saying xsltproc was
not found than trying to execute the argument to xsltproc. Now message is:

  XSLT     depmod.d.5
  /bin/sh: line 1: xsltproc: command not found

Instead of:

  XSLT     depmod.d.5
  /bin/sh: --nonet: command not found