Interoperability of complex types used in LAPACK and (GNU/Intel) BLAS headers with C++ code

[julianh-siemens]

The LAPACK and (GNU/Intel) BLAS headers in FlexiBLAS use the complex data types in the C style, like "float/double complex".
When using these headers in C++ code this can lead to issues since the C++ <complex> header undefines the complex macro:

// Get rid of a macro possibly defined in <complex.h>
#undef complex

The reference LAPACK implementation works around this issue by including different headers depending on which language is used and then defines its own lapack_complex_float and lapack_complex_double types https://github.com/Reference-LAPACK/lapack/blob/master/LAPACKE/include/lapack.h#L43-L77