configure: msvc: Set the target windows version to XP if no target is set

MSVC 2010 (or more precisely, Windows SDK 7.0 which comes with MSVC
2010) sets _WIN32_WINNT to the constant for Windows 7 if nothing is
set. This could lead to the libav configure script detecting and
using functions only present in Windows 7 or newer, which in most
cases isn't desired. If the caller explicitly wants this, the caller
can add the _WIN32_WINNT define via --extra-cflags, setting the desired
version.

Signed-off-by: Martin Storsjö <martin@martin.st>
win32: Do not use GetProcAddress when compiled for Vista+

Instead link directly to the condition variable functions.

Signed-off-by: Martin Storsjö <martin@martin.st>
vorbis: refactor vorbis_residue_decode_internal
Merge commit '0b6899f154347cdd0aa66c5ec16ed9c65871766f'

* commit '0b6899f154347cdd0aa66c5ec16ed9c65871766f':
  configure: msvc: Set the target windows version to XP if no target is set

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'c360fb174b01170ab3fd3048d4d0acb44aa4e834'

* commit 'c360fb174b01170ab3fd3048d4d0acb44aa4e834':
  win32: Do not use GetProcAddress when compiled for Vista+

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '98186578a2a21af5bfe6dd71d222dc270f763c7d'

* commit '98186578a2a21af5bfe6dd71d222dc270f763c7d':
  vorbis: refactor vorbis_residue_decode_internal

Conflicts:
	libavcodec/vorbisdec.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/filters: extend/fix documentation for the geq filter
doc/filters: add RGB geq example