simd instructions for complex stencil not working for all versions of compilers

[xuqimen]

The ENABLE_SIMD_COMPLEX option in the makefile can bring significant speedup for systems with k-points. However, it only brings speedup for relatively new versions of compilers. For some old compilers, it might even give wrong results. Therefore the user would need to test by turning on and off the ENABLE_SIMD_COMPLEX option (and check both the answers and the speed) to confirm whether the compiler is compatible with this option. This option is off by default.