title: Compiler detects allegro.h, but not allegro_primitives.h
body: <p>When I compile my Allegro 5 program using g++, it complains about <code>undefined reference to 'al_init_primitives_addon', al_draw_filled_rectangle</code> and other such functions that are found in allegro_primitives.h. It does not complain about functions in allegro.h like <code>al_create_display</code>.</p>
  
  <p>Includes:</p>
  
  <pre><code>#include &lt;allegro5/allegro.h&gt;
  
  #include &lt;allegro5/allegro_primitives.h&gt;
  
  #include "objects.h"
  
  #include "main.h"
  </code></pre>
  
  <p>Compiler command:</p>
  
  <pre><code>g++ main.cpp -o game -lallegro -I/usr/include/allegro5 -L/usr/lib/allegro5
  </code></pre>
  
  <p>Complaints:</p>
  
  <pre><code>/tmp/ccAyQlcl.o: In function `main':
  main.cpp:(.text+0xef): undefined reference to `al_init_primitives_addon'
  /tmp/ccAyQlcl.o: In function `Draw()':
  main.cpp:(.text+0x38c): undefined reference to `al_draw_filled_rectangle'
  main.cpp:(.text+0x415): undefined reference to `al_draw_filled_rectangle'
  </code></pre>
  
  <p>MSVC++ compiles this fine, by the way.</p>