configure: prettify atomics handling.

Add simpler names and a shorthand for native atomics (as opposed to
pthreads fallback).
configure: Check for the atomic.h functions used in the suncc atomics header

The "suncc" atomics implementation uses a suncc specific memory
barrier, but also relies on a few atomic functions from atomic.h,
that are not suncc specific but specific to solaris. This made
the current implementation fail on suncc on linux.
Merge commit 'f9f6402e9c9ce3642df981b48507df3a2d956f65'

* commit 'f9f6402e9c9ce3642df981b48507df3a2d956f65':
  configure: prettify atomics handling.

Conflicts:
	configure

Merged-by: Michael Niedermayer <michaelni@gmx.at>
smacker: fix off by one error

Regression since a93b572ae4f517ce0c35cf085167c318e9215908.

Fixes #2426.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Merge commit '817dff578f13ce97576c609ab141508b9dc782e9'

* commit '817dff578f13ce97576c609ab141508b9dc782e9':
  configure: Check for the atomic.h functions used in the suncc atomics header

Conflicts:
	configure

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Fix make checkheaders without opencl headers.