ao_dsound: add missing include

libavutil/common.h is needed for FF_ARRAY_ELEMS.
configure: check for presence of glext.h

vo_opengl depends on glext.h to build. Also link to Khronos' copy, which
should work on all compilers and is kept up-to-date with newer
extensions.
w32_common: Cygwin64 fixes

Good news: MPV worked fine even without the fixes, but pointer size
mismatch warnings aren't the nicest things to leave lying around.

Fix macro that assumed HWND is uint32_t-sized.

Win64 is also a special butterfly and is an LLP64 platform on amd64
CPUs, while all the other amd64 platforms are LP64. Cygwin decided to go
with the other platforms, and thus sizeof(long) != sizeof(int), and in
cygwin's windows headers LONG is int-sized. Fix an mp_msg that assumed
LONG is long.