GameUpdateAndRender in a separate file causes issues

build.sh

#!/usr/bin/env bash

mkdir -p build

gcc -framework Cocoa \
  -x objective-c \
  code/osx_handmade.mm \
  -o build/handmade

error.log

In file included from code/osx_handmade.mm:3:
code/handmade.h:3:15: warning: function 'GameUpdateAndRender' has internal linkage but is not
      defined [-Wundefined-internal]
internal void GameUpdateAndRender();
              ^
code/osx_handmade.mm:63:3: note: used here
  GameUpdateAndRender();
  ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_GameUpdateAndRender", referenced from:
      _main in osx_handmade-4f9411.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [osx] Error 1

handmade.cpp

#include "handmade.h"

internal void GameUpdateAndRender() {
}

handmade.h

#define internal static

internal void GameUpdateAndRender();

osx_handmade.mm

#include <Cocoa/Cocoa.h>

#include "handmade.h"

void OSXInitNSApplication(void)
{
    NSApplication *app = [NSApplication sharedApplication];
    [NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
}

void OSXCreateMainMenu() {
  NSString* appName = @"Handmade Hero";

  NSMenu* menubar = [NSMenu new];
  NSMenuItem* appMenuItem = [NSMenuItem new];

  [menubar addItem:appMenuItem];
  [NSApp setMainMenu:menubar];

  NSMenu* appMenu = [NSMenu new];

  NSString* fullScreenTitle = @"Toggle Full Screen";
  NSMenuItem* fullScreenItem = [[NSMenuItem alloc]
    initWithTitle:fullScreenTitle
    action:@selector(toggleFullScreen:)
    keyEquivalent:@"f"];
  [appMenu addItem:fullScreenItem];

  NSString* quitTitle = [@"Quit " stringByAppendingString:appName];
  NSMenuItem* quitMenuItem = [[NSMenuItem alloc]
    initWithTitle:quitTitle
    action:@selector(terminate:)
    keyEquivalent:@"q"];
  [appMenu addItem:quitMenuItem];

  [appMenuItem setSubmenu:appMenu];
}

void OSXCreateWindow() {
  NSString* appName = @"Handmade Hero";
  NSRect initialFrame = NSMakeRect(0, 0, 640, 480);

  NSWindow* window = [[NSWindow alloc] initWithContentRect:initialFrame
    styleMask:NSTitledWindowMask
    backing:NSBackingStoreBuffered
    defer:NO];

  [window cascadeTopLeftFromPoint:NSMakePoint(20,20)];
  [window setBackgroundColor:NSColor.blackColor];
  [window setTitle:appName];
  [window makeKeyAndOrderFront:nil];

  [NSApp activateIgnoringOtherApps:YES];
  [NSApp run];
}

int main (int argc, char const *argv[])
{
  OSXInitNSApplication();
  OSXCreateMainMenu();
  OSXCreateWindow();

  // Commenting out this call, allows the program to compile
  // and run again.
  GameUpdateAndRender();

  return 0;
}