[CM] s7_complex in C++

[bil at ccrma.Stanford.EDU]

Good grief, what a nightmare.  It looks to me like s7 can be compiled
by gcc/clang (C compilers), and g++ (I tried version 12.2.0 and
14.2.1), but not by clang++ (tried versions 12.0.0 and 18.1.8).
Getting gcc/g++ to work with complex numbers was tricky, but
clang++ has apparently decided to make it impossible.  I'll have
to poke around online to see what the work-around might be.
For now, it might work to comment out the s7.h entries for
the complex vector functions (but I ran into some other problem
when I tried that).  Also I had to change noreturn to no_return
for clang++.