title: Building libgc fails on Mac OS X 10.6 Snow Leopard: ... and require _XOPEN_SOURCE to be defined
body: <p>The following steps:</p>
  
  <pre><code>$ wget http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc-7.0.tar.gz
  $ tar xvfz gc-7.0.tar.gz
  $ cd gc-7.0
  $ ./configure
  $ make
  </code></pre>
  
  <p>fail with this error:</p>
  
  <pre><code>In file included from mach_dep.c:163:
  /usr/include/ucontext.h:42:2: error: #error ucontext routines are deprecated, 
  and require _XOPEN_SOURCE to be defined
  </code></pre>