Undefined Reference? But I've already implemented the function
===

**display.h**

    #ifndef PRO_DISPLAY_H
    #define PRO_DISPLAY_H
    
    /** Initializes the display **/
    int pro_display_init(void);
    
    #endif /* PRO_DISPLAY_H */


**display.c**

    #include "main.h"
    
    static int height_ = 300;
    static int width_ = 300;
    static int bpp_ = 16;
    
    static SDL_Surface* screen_ = NULL;
    
    int pro_display_init(void)
    {
        screen_ = SDL_SetVideoMode(width_, height_, bpp_, SDL_HWSURFACE|SDL_DOUBLEBUF);
        if (!screen_)
        {
            pro_sdl_error("Video initialization failed.");
            return 0;
        }
    
        return 1;
    }

**main.h**

    #ifndef PRO_MAIN_H
    #define PRO_MAIN_H
    
    // standard headers
    #include <stdlib.h>
    #include <stdlib.h>
    
    // conditional headers
    #if defined(WIN32) || defined(_WIN32)
    #include <windows.h>
    #endif
    
    // our own headers
    #include "scripter.h"
    #include "ttf_util.h"
    #include "events.h"
    #include "display.h"
    
    // some macros
    #define pro_error(...) fprintf(stderr, __VA_ARGS__)
    #define pro_sdl_error(x) fprintf(stderr, "%s. \n=> %s\n", x, SDL_GetError())
    #define pro_ttf_error(x) fprintf(stderr, "%s. \n=> %s\n", x, TTF_GetError())
    
    #endif /* PRO_MAIN_H */

** main.c**

    #include "main.h"
    
    int main(int argc, char* argv[])
    {
        pro_display_init();
        return 0;
    }

The Error:

    main.c|5|undefined reference to `pro_display_init()'|

Checked the build process. Made sure I was adding "display.c" to gcc's input files. I'm at my wit's end. Why the error?