Made appropriate changes to be able to successfully
build C++ files using a Visual C++ build on Windows.

Based on an earlier patch by Kyle Schwarz.

Comments:

-- compat/w32pthreads.h: Made appropriate changes to w32pthreads.h to
   get it to build when it is being included in a C++ file and built
   with Visual C++.  This is mostly a copy of Kyle Schwarz's patch as
   described above.

-- configure:
a) Now calling set_ccvars CXX to cause the various CXX_ variables to
   be setup properly.  For example, with MSVC (Microsoft Visual C++),
   this causes CXX_O to be set to -Fo$@ instead of using the default
   value.  The default value does not work with Visual C++.  This
   change will also have the impact of correcting CXX_O (and possibly
   CXX_C) for other compilers, although this is really only relevant
   for the Intel compiler, in addition to MSVC.
b) Now using cl for the C++ compiler for the MSVC toolchain.  This is
   currently only relevant for building the
   Blackmagic/Decklink-related files under avdevice.
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>