Improve performance of GEMM for small matrices when SMP is defined.

Always checking num_cpu_avail() regardless of whether threading will actually
be used adds noticeable overhead for small matrices.  Most other uses of
num_cpu_avail() do so only if threading will be used, so do the same here.

Use a single thread for small input size in zaxpy

Use usleep instead of sched_yield by default

copies daxpy improvement from #27, see #1560

sched_yield only burns cpu cycles, fixes #900,  see also #923, #1560

Initial support for SkylakeX / AVX512

Improved AVX512 test case for c_check

clang 3.4 managed to accept the original test code, only to fail on the actual Skylake asm later

Restore _Atomic define before stdatomic.h for old gcc

see #1593

Check build system support for AVX512 instructions

Fix inverted condition in _Atomic declaration

fixes #1593

Disable AVX512 (Skylake X) support if the build system is too old

this required switching to the generic gemm_beta code (which is faster anyway on SKX)
for both DGEMM and SGEMM

Performance for the not-retuned version is in the 30% range

This patch adds the basic infrastructure for adding the SkylakeX (Intel Skylake server)
target. The SkylakeX target will use the AVX512 (AVX512VL level) instruction set,
which brings 2 basic things:
1) 512 bit wide SIMD (2x width of AVX2)
2) 32 SIMD registers (2x the number on AVX2)

This initial patch only contains a trivial transofrmation of the Haswell SGEMM kernel
to AVX512VL; more will follow later but this patch aims to get the infrastructure
in place for this "later".

Full performance tuning has not been done yet; with more registers and wider SIMD
it's in theory possible to retune the kernels but even without that there's an
interesting enough performance increase (30-40% range) with just this change.

Revert "take out unused variables"

This reverts commit e5752ff9b3.

The variables i and n are used in the `#if !__GLIBC_PREREQ(2, 7)`
branch.

Closes gh-1586.

Fixes from Lapack-Reference PR 253

When minimal workspace is given in ?hesv_aa, ?sysv_aa, ?hesv_aa_2stage, ?sysv_aa_2stage, now no error is given
Quick return for ?laqr1

LAPACKE interfaces for Aasen's functions now call ?sytrf_aa and ?hetrf_aa instead of ?sytrf and ?hetrf

Work around name clash with Windows10's winnt.h

Handle INCX=0,INCY=0 case

Update version number on the develop branch to 0.3.1.dev

Fix paths to LIN and EIG tests