set_icon.c

// > cl /LD set_icon.c user32.lib
// :call libcallnr('path/to/set_icon.dll', 'set_icon', v:windowid . ',' . 'path/to/myvim.ico')
#include <windows.h>
#include <stdio.h>

__declspec(dllexport) int set_icon(char **args)
{
    HWND hWnd;
    char iconpath[1024];
    HICON hIcon;

    if (sscanf(args, "%d,%s", &hWnd, &iconpath) != 2)
        return -1;
    hIcon = (HICON)LoadImage(NULL, iconpath, IMAGE_ICON, 0, 0, LR_LOADFROMFILE);
    if (hIcon == NULL)
        return -2;
    SendMessage(hWnd, WM_SETICON, ICON_BIG, hIcon);
    SendMessage(hWnd, WM_SETICON, ICON_SMALL, hIcon);
    return 0;
}