title: Check if a command returns nothing in Makefile
body: <p>For a C library, I need to check whether the current compiler is for x86_64 due to memory issues. The command I found that exactly meets my requirement:</p>
  
  <pre><code>CXXARCH:=$(${CXX} -dumpmachine | grep -i 'x86_64')
  </code></pre>
  
  <p>where <code>${CXX}</code> is either <code>gcc</code> or <code>clang</code>. For an <code>x86_64</code> machine, this will return a non-empty string. For a 32-bit machine, say Raspberry Pi, this will be empty.</p>
  
  <p>how can I distinguish between the two cases?</p>
  
  <p>I did this:</p>
  
  <pre><code>ifneq (${CXXARCH},)
  MAGICVAR:=-DMY_DEFINE
  endif
  </code></pre>
  
  <p>With an <code>$(info)</code> print, I ensured that in a Raspberry Pi, this condition is not being fulfilled, which it should, because the command <code>clang-6.0 -dumpmachine</code> returns: <code>armv7l-unknown-linux-gnueabihf</code>. So why is this condition not being executed? What am I doing wrong?</p>