Fix a bunch of minor build and test issues.

• SPQRSupport included a header with the wrong relative path
• minmax visitor is only vectorized if vectorized comparisons are available
• half_float/bfloat16_float included a superfluous internal header
• gpu_basic/gpu_example didn't properly define EIGEN_USE_GPU, which meant GPU packets weren't actually used
• incomplete_cholesky/sparselu included unused "unsupported" headers
• sparse_extra unnecessarily duplicated the original sparse_product test