A _very_ naive makefile implementation that suits my purposes.

make.rb

#!/usr/bin/env ruby

# @author Simon (psyomn) Symeonidis
# Because doing something this simple in a Makefile requires you to read a huge
# fucking manual.

def make(note, output_name)
   puts "#{note} >>> #{output_name}"
   `pandoc #{note} -o #{output_name}`
end

notes = Dir['*.markdown']
outdir = "pdf/"

notes.each do |note|

output_name = note.split('.')[0..-2].join('.').concat('.pdf')
output_name = "#{outdir}#{output_name}"

# File does not exist, or is not updated
if !File.exist? output_name
  make(note, output_name)
elsif File.mtime(output_name).to_i < File.mtime(note).to_i
  make(note, output_name)
else 
  puts "Skip #{note}"
end
end