check knitr version before converting Rmd to md

on a suggestion of @sritchie73 and @aammd

Makefile

@@ -62,7 +62,8 @@ motivation.html : motivation.md _layouts/slides.revealjs Makefile
 	    -o $@ $<
 
 # Pattern to convert R Markdown to Markdown.
-%.md: %.Rmd $(R_CHUNK_OPTS)
+%.md: %.Rmd $(R_CHUNK_OPTS) tools/check_knitr_version.R
+	Rscript -e "source('tools/check_knitr_version.R')"
 	Rscript -e "knitr::knit('$$(basename $<)', output = '$$(basename $@)')"
 
 ## commands : Display available commands.

tools/check_knitr_version.R

+if (require("knitr")) {
+    if (packageVersion("knitr") < '1.9.19') {
+        stop("knitr must be version 1.9.20 or higher")
+    }
+} else stop("knitr 1.9.20 or above is needed to build the lessons.")