fix: Wasn't missing lib - it's GL/gl.h on linux

.travis.yml

@@ -14,7 +14,6 @@ matrix:
             - libc6-i386
             - libcurl4-openssl-dev:i386
             - libpng12-dev:i386
-            - libgl1-mesa-dev:i386
 
     - os: osx
       compiler: clang

main/source/particles/opengl.cpp

@@ -11,10 +11,15 @@
 #include <windows.h>
 #include <GL/gl.h>
 #else
-#include <OpenGL/gl.h>
 #define NULL 0
 #endif
 
+#ifdef __APPLE__
+#include <OpenGL/gl.h>
+#else
+#include <GL/gl.h>
+#endif
+
 // XXX #include <iostream.h>
 
 // Emit OpenGL calls to draw the particles. These are drawn with