Add variable to represent Pandoc

Pandoc had some changes in the last versions.
Having a variable for Pandoc will help
maintainers to handle different versions of Pandoc.

Makefile

+PANDOC ?= pandoc
+
 # R Markdown files.
 SRC_RMD = $(wildcard ??-*.Rmd)
 DST_RMD = $(patsubst %.Rmd,%.md,$(SRC_RMD))
@@ -43,13 +45,13 @@ preview : $(DST_ALL)
 
 # Pattern for slides (different parameters and template).
 motivation.html : motivation.md _layouts/slides.revealjs Makefile
-	pandoc -s -t revealjs --slide-level 2 \
+	${PANDOC} -s -t revealjs --slide-level 2 \
 	--template=_layouts/slides \
 	-o $@ $<
 
 # Pattern to build a generic page.
 %.html : %.md _layouts/page.html $(FILTERS)
-	pandoc -s -t html \
+	${PANDOC} -s -t html \
 	--template=_layouts/page \
 	--filter=tools/filters/blockquote2div.py \
 	--filter=tools/filters/id4glossary.py \
@@ -66,6 +68,7 @@ commands : Makefile
 
 ## settings : Show variables and settings.
 settings :
+	@echo 'PANDOC:' $(PANDOC)
 	@echo 'SRC_RMD:' $(SRC_RMD)
 	@echo 'DST_RMD:' $(DST_RMD)
 	@echo 'SRC_MD:' $(SRC_MD)