Add PDFLATEX var to var.mk

This is not strictly necessary any more than some of the others but two
entries use this tool (at least since I added them back - they were
missing) and it seems better to have it this way.

2018/poikola/Makefile

@@ -146,7 +146,7 @@ data: ${DATA}
 	@${TRUE}
 
 docs: ${DOCS}
-	pdflatex ${DOCS}
+	${PDFLATEX} ${DOCS}
 
 
 # both all and alt