[Bf-committers] MinGW support

[Sergey Sharybin]

Hey everyone,

There are some missing precompiled libraries (sndfile, opensubdiv, OSL) and
some misng libraries update (python, SDL, OpenAL)  for MinGW.

This makes MinGW builds quite diverged from the official builds feature-set
and lib-version wise, moving MinGW builds to the boundary where we can't
call it officially supported anymore.

Additioanlly, svn:externals for mingw folders seems to be misconfigured now
and needs clean up.

Anyone to have a look and fix all those issues?

-- 
With best regards, Sergey Sharybin