GCC 4.4.0 complains:
gstglwindow_win32.c: In function 'gst_gl_window_set_external_window_id':
gstglwindow_win32.c:207: error: cast from pointer to integer of different size

The offending line is:
SetWindowLongPtr ((HWND) id, GWL_WNDPROC, (DWORD) (guint64) sub_class_proc);

Prototype for SetWindowLongPtr is:
WINUSERAPI LONG_PTR WINAPI SetWindowLongPtrW(HWND,int,LONG_PTR); (on x64)
or
WINUSERAPI LONG WINAPI SetWindowLongW(HWND,int,LONG); (on x86)

In MinGW LONG_PTR is defined as
typedef __int64 LONG_PTR, *PLONG_PTR; (on x64)
or as
typedef  long LONG_PTR, *PLONG_PTR; (on x86)

and LONG is defined as
typedef long LONG;

So, on x86 systems the SetWindowLongPtr accepts LONG, LONG_PTR being equivalent to LONG, while on x64 systems it accepts LONG_PTR itself. Both DWORD and guint64 are out of place.

In case you're wondering, MSDN says that
typedef __int3264 LONG_PTR; 
which is the same thing.

How to fix:
replace "(DWORD) (guint64)" with "(LONG_PTR)"