build vpx on win32

[Antoine Martin]

r734 adds the hooks to specify the right paths, but now it fails with (multi-line cl.exe line edited by hand so as not to overflow the bug tracker window):

building 'xpra.vpx.codec' extension
creating build\temp.win32-2.7\Release\xpra\vpx
C:\Program Files\Microsoft Visual Studio 9.0\VC\BIN\cl.exe /c \
    /nologo /Ox /MD /W3 /GS- /DNDEBUG -Ixpra/vpx/win32 -IZ:\vpx-vp8 \
    debug-src-x86-win32mt-vs9-v1.0.0/include -IZ:\ffmpeg-win32-shared/include \
    -IC:\Python27\include -IC:\Python27\PC /Tcxpra/vpx\codec.c \
    /Fobuild\temp.win32-2.7\Release\xpra/vpx\codec.obj \
    codec.c
z:\ffmpeg-win32-shared\include\libavutil\common.h(72) : error C2054: expected '(' to follow 'inline'
z:\ffmpeg-win32-shared\include\libavutil\common.h(73) : error C2085: 'av_log2_c' : not in formal parameter list
z:\ffmpeg-win32-shared\include\libavutil\common.h(73) : error C2143: syntax error : missing ';' before '{'
z:\ffmpeg-win32-shared\include\libavutil\common.h(88) : error C2054: expected '(' to follow 'inline'
z:\ffmpeg-win32-shared\include\libavutil\common.h(89) : error C2085: 'av_log2_16bit_c' : not in formal parameter list
(...)

All you need to do to try to build is to set vpx_PATH in setup.py (download links therein)

[Antoine Martin]

r735 + r736 + r737 fixes it all

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/112