title: Xcode 4.3.3 can't find any header files
body: <p>Up until now I've managed to get Xcode to link to the Gnu Scientific Libraries (GSL) which I've installed under /usr/local/lib/ and with header files under /usr/local/include. Under "Build Phases" > "Link Binary With Libraries" I had added libgsl.a etc. Today, Xcode gives an error message claiming it can't find header files.</p>
  
  <p>For example</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;gsl/gsl_matrix.h&gt;
  
  int main(int argc, const char * argv[])
  {
  
      printf("Hello, World!\n");
      return 0;
  }
  </code></pre>
  
  <p>results in 'gsl/gsl_matrix.h' file not found. I've tried to change the User Header Search path to no avail. Using gcc main.c -lgsl -lgslcblas on the command line works perfectly. Could anyone tell me what exactly one needs to do to get this to work in Xcode? Also, why has it suddenly stopped working for no apparent reason.</p>
  
  <p>Many kind thanks.</p>