commands

replaces obsolete (no longer used anywhere AFICT check_compiler_gcc4
with some more general utilities for checking GCC version

I found that when building the latest master branch on Cygwin, while testing #16246, that thousands of warnings were generated at build time like:

numpy/core/src/npysort/binsearch.c.src: In function ‘binsearch_left_bool’:
numpy/core/src/npysort/binsearch.c.src:82:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
Granted this is just a warning, so I don't think it's a serious issue.

It seems the test that was supposed to check for __attribute__ support was not working as expected. The #pragmas only take effect if I provide a function body--they are ignored for bare declarations. I don't know if that's by intent, or if it's a GCC issue. For reference:

$ gcc --version
gcc (GCC) 7.4.0

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

This makes it easier to visually jump between functions.

A couple places have changed to not emit leading whitespace where they previously did. Since this is C code and not fortran, that doesn't matter.

1) use __builtin_cpu_supports("avx512f") only for gcc ver >= 5

2) Introduced two new macro's:
HAVE_ATTRIBUTE_TARGET_@ISA@_WITH_INTRINSICS for ensuring compiler can
compile functions that use intrinsics and are compiled with avx2/avx512f
attributes

closes gh-6427

Define it as NPY_RESTRICT
Restrict indicates a memory block does not alias, gcc supports it in c89
with the __restrict__ keyword.