In many cases, only one of std::complex<double> and std::complex<float> is
used, so while we continue to define typemaps for both by default, for
compatibility with the other modules, add a possibility to avoid generating
the code for the unwanted specialization by predefining the corresponding
SWIG_NO_STD_COMPLEX_$TYPE before including this file.
For consistency with the other modules (Python, Ruby and JS), allow including
<std_complex.i> directly when using C# too instead of including <complex.i>
which only works in C++ mode (i.e. when using std::complex) anyhow with C# for
now.
