[CM] s7_complex does not compile for MSVC

[bil at ccrma.Stanford.EDU]

Thanks for the info about MSVC.  I think I could get
further in clang++, assuming C++11, with

     using s7_complex = std::complex<double>;

but there were still endless complaints (mostly about
missing casts).  In clang++, the first warning is

    clang: warning: treating 'c' input as 'c++' when in C++ mode, this 
behavior is deprecated

which makes me think the C++ community has decided to
cut its ties to C.  I'll go back and poke at this,
but it feels like a hopeless situation.  Is there
some way in MSVC to ask for C and not C++?