Use pdflatex instead of plain latex

Use pdflatex instead of plain latex

With recent texlive, I'm running into the following problem:

[snip]
! Package ifpdf Error: Name clash, \ifpdf is already defined.

See the ifpdf package documentation for explanation.
? H
Incompatible versions of \ifpdf can cause problems,
therefore package loading is aborted.
[snap]

Since we're only building pdf output anyway, I'm getting rid of it
and use `pdflatex' instead of `latex' in the Makefile.