Implement multiple rules per target

[sjackman]

See https://www.gnu.org/software/make/manual/html_node/Multiple-Rules.html

In the following example, biomake does not create the file data.tsv.

all: report.html

report.rmd:
	printf '```{r}\nlibrary(ggplot2)\nlibrary(readr)\nggplot(read_tsv("data.tsv"), aes(x=x, y=y)) + geom_point()\n```\n' >$@

data.tsv:
	printf "x\ty\n1\t1\n2\t4\n3\t9\n" >$@

%.html: %.rmd
	Rscript -e 'rmarkdown::render("$<", "html_document", "$@")'

report.html: data.tsv

❯❯❯ make -n
printf '```{r}\nlibrary(ggplot2)\nlibrary(readr)\nggplot(read_tsv("data.tsv"), aes(x=x, y=y)) + geom_point()\n```\n' >report.rmd
printf "x\ty\n1\t1\n2\t4\n3\t9\n" >data.tsv
Rscript -e 'rmarkdown::render("report.rmd", "html_document", "report.html")'
❯❯❯ biomake -n
  printf '```{r}\nlibrary(ggplot2)\nlibrary(readr)\nggplot(read_tsv("data.tsv"), aes(x=x, y=y)) + geom_point()\n```\n' >report.rmd
 Rscript -e 'rmarkdown::render("report.rmd", "html_document", "report.html")'
Target all not materialized - build required
❯❯❯ biomake
…
Error: 'data.tsv' does not exist in current working directory

Execution halted
While building report.html: Error 1 executing Rscript -e 'rmarkdown::render("report.rmd", "html_document", "report.html")'