title: Check if a program exists from a Makefile
body: <p>How can I check if a program is callable from a Makefile?</p>
  
  <p>(That is, the program should exist in the path or otherwise be callable.) </p>
  
  <p>It could be used to check for which compiler is installed, for instance.</p>
  
  <p>E.g. something like <a href="https://stackoverflow.com/questions/592620/check-if-a-program-exists-from-a-bash-script">this question</a>, but without assuming the underlying shell is POSIX compatible.</p>