Build system: Improvements for ./configure script

Build system: Improvements for ./configure script

- Make sure that always a valid PDF-to-PostScript renderer is
  selected, especially if one or another renderer is disabled, like
  "hybrid" ("--with-pdftops=hybrid") does not make sense if Ghostscipt
  is disabled ("--disable ghostscript"). In this case the first valid
  renderer found will beused.

- In the summary at the end report which PDF-to-PostScript renderer
  gets actually used.

- Changed oder of treating the PDF-to-PostScript renderer, so that as
  first choice Ghostscript would be taken and as second choice
  Poppler's pdftops.

- Generate for each supported PDF renderer a "HAVE_..." macro in
  config.h, for easily excluding unnecessary code.