double RecordFPS()
{
   /* Source: http://cone3d.gamedev.net */

#define FPS_BUCKET_SIZE 20
#define FPS_AVERAGE_TIME 1000

   static unsigned int bucket[FPS_BUCKET_SIZE];
   static int now = 0, end = 0;
   int FPS = 0;
   unsigned int tick = glutGet(GLUT_ELAPSED_TIME);

   /* Add current time to bucket */
   end++;
   if (end == FPS_BUCKET_SIZE)
      end = 0;
   bucket[end] = tick;

   /* If too high FPS, need a safe wrap around so FPS won't jump.
    * Bug from original source would have the time difference jump
    * down to near 0, causing the FPS for a couple of frames to show
    * up incorrectly by not using the whole bucket to search.
    */
   if (end == now)
   {
      now = end + 1;
      if (now == FPS_BUCKET_SIZE)
         now = 0;
   }

   /* Loop through till we find time of at least X seconds ago */
   if (tick < FPS_AVERAGE_TIME)
      tick = 0;
   else
      tick -= FPS_AVERAGE_TIME;

   while (bucket[now] < tick)
   {
      now++;
      if (now == FPS_BUCKET_SIZE)
         now = 0;
   }

   /* How much difference is between 'now' and 'end' is how many frames drawn in last 10 seconds */
   FPS = end - now;
   if (end < now)
      FPS += FPS_BUCKET_SIZE;

   if (end == now)
      return 0.0;

   return FPS * 1000.0 / (double)(bucket[end] - bucket[now]);
}

brad@brad-laptop:~/Desktop/pearlharbour/pearlharbour$ make
gcc -O2 -s -DNDEBUG -Wall -fomit-frame-pointer main.o util.o graphics.o game.o -o pearlharbour -lSDL
main.o: In function `main':
main.c:(.text+0x19b): undefined reference to `Game_DrawShips'
main.c:(.text+0x1a1): undefined reference to `Game_DrawMap'
main.c:(.text+0x1a6): undefined reference to `Game_DrawWeapons'
main.c:(.text+0x1ab): undefined reference to `Game_DrawPlanes'
main.c:(.text+0x1b1): undefined reference to `Game_DrawAnims'
game.o: In function `Game_DrawRadar':
game.c:(.text+0xc0): undefined reference to `mapobjectset'
game.c:(.text+0x16a): undefined reference to `shipobjectset'
game.c:(.text+0x21c): undefined reference to `planeset'
game.o: In function `Game_Update':
game.c:(.text+0x475): undefined reference to `UpdatePlanes'
game.c:(.text+0x47a): undefined reference to `UpdateWeapons'
game.c:(.text+0x47f): undefined reference to `UpdateFlak'
game.c:(.text+0x484): undefined reference to `UpdateShips'
game.c:(.text+0x489): undefined reference to `UpdateAnims'
game.o: In function `TryFireSecondary':
game.c:(.text+0x59e): undefined reference to `AddWeapon'
game.o: In function `TryFirePrimary':
game.c:(.text+0x6f0): undefined reference to `AddWeapon'
game.o: In function `CheckWin':
game.c:(.text+0x760): undefined reference to `SpawnFighters'
game.c:(.text+0x77f): undefined reference to `planeset'
game.c:(.text+0x795): undefined reference to `SpawnBombers'
game.o: In function `Game_CleaupLevel':
game.c:(.text+0x7ae): undefined reference to `planeset'
game.c:(.text+0x7ba): undefined reference to `planeset'
game.c:(.text+0x7ce): undefined reference to `mapobjectset'
game.c:(.text+0x7da): undefined reference to `mapobjectset'
game.c:(.text+0x7ee): undefined reference to `shipobjectset'
game.c:(.text+0x7fa): undefined reference to `shipobjectset'
game.c:(.text+0x80e): undefined reference to `flakobjectset'
game.c:(.text+0x81a): undefined reference to `flakobjectset'
game.c:(.text+0x84e): undefined reference to `animset'
game.c:(.text+0x85a): undefined reference to `animset'
game.o: In function `Game_LoadLevel':
game.c:(.text+0x87b): undefined reference to `planeset'
game.c:(.text+0x88f): undefined reference to `mapobjectset'
game.c:(.text+0x89b): undefined reference to `shipobjectset'
game.c:(.text+0x8a7): undefined reference to `flakobjectset'
game.c:(.text+0x8bf): undefined reference to `animset'
game.c:(.text+0xc5e): undefined reference to `AddShip'
game.c:(.text+0xc93): undefined reference to `AddShip'
game.c:(.text+0xcc3): undefined reference to `AddShip'
game.c:(.text+0xcf5): undefined reference to `AddShip'
game.c:(.text+0xcfd): undefined reference to `AddFlakProtectShip'
game.c:(.text+0xd2d): undefined reference to `AddShip'
game.c:(.text+0xd5d): undefined reference to `AddShip'
game.c:(.text+0xd8f): undefined reference to `AddShip'
game.c:(.text+0xd97): undefined reference to `AddFlakProtectShip'
game.c:(.text+0xdc7): undefined reference to `AddShip'
game.c:(.text+0xde9): undefined reference to `AddIsland'
game.c:(.text+0xe09): undefined reference to `AddFlakEmplacement'
game.c:(.text+0xe1d): undefined reference to `AddJungle'
game.c:(.text+0xe38): undefined reference to `AddIsland'
game.c:(.text+0xe4c): undefined reference to `AddJungle'
game.c:(.text+0xe60): undefined reference to `AddJungle'
game.c:(.text+0xe74): undefined reference to `AddIsland'
game.c:(.text+0xe94): undefined reference to `AddFlakEmplacement'
game.c:(.text+0xeb4): undefined reference to `AddFlakEmplacement'
game.c:(.text+0xec7): undefined reference to `SpawnEnemies'
game.c:(.text+0xf1f): undefined reference to `AddPlane'
game.c:(.text+0xf69): undefined reference to `AddPlane'
game.c:(.text+0xfbb): undefined reference to `AddPlane'
game.c:(.text+0x1005): undefined reference to `AddPlane'
collect2: ld returned 1 exit status
make: *** [pearlharbour] Error 1