title: gmp.h missing when trying to use boost library
body: <p>I am trying to use the boost library with QT on windows. I've successfully build the library and also managed to include it in my project. However, on including gmp (<code>#include "boost/multiprecision/gmp.hpp"</code>) and creating an object (<code>boost::multiprecision::mpz_int myint;</code>) I get the following error:</p>
  
  <p><code>C:\Users\Laurenz\Documents\libraries\boost_1_66_0\include\boost\multiprecision\gmp.hpp:31: error: gmp.h: No such file or directory</code></p>
  
  <p>And indeed, I haven't been able to find any such file in the boost directory. What did I do wrong?</p>