Set AUTOCONF variable when maintainer-mode is not used

Fixes build issue introduced in 8894bad3 when configure.ac is altered
and maintainer-mode is not enabled.
Minor code cleanup in s_sin.c

 - Remove redundant mynumber union definitions
 - Clean up a clumsy ternary operator
 - Rename TAYLOR_SINCOS to TAYLOR_SIN since we're only expanding the
   sin Taylor series in it.
manual/memory.texi: Bring aligned allocation docs up to date.

The current documentation suggests using memalign and valloc which
are now considered obsolete, so suggest using posix_memalign instead.
Also document the possible error return and errno values for memalign
and posix_memalign and improve documentation of __memalign_hook.

ChangeLog:

2013-12-16  Will Newton  <will.newton@linaro.org>

	* manual/memory.texi (Malloc Examples): Clarify default
	alignment documentation.  Suggest posix_memalign rather
	than memalign or valloc.
	(Aligned Memory Blocks): Remove suggestion to use memalign
	or valloc.  Remove obsolete comment about BSD.
	Document memalign errno values and mark the function obsolete.
	Document posix_memalign returned error codes.  Mark valloc
	as obsolete.  (Hooks for Malloc): __memalign_hook is also
	called for posix_memalign and valloc.
	(Summary of Malloc): Add posix_memalign to function summary.
	__memalign_hook is also called for posix_memalign and valloc.
manual/memory.texi: Document aligned_alloc.

ChangeLog:

2013-12-16  Will Newton  <will.newton@linaro.org>

	* manual/memory.texi (Malloc Examples): Mention aligned_alloc.
	(Aligned Memory Blocks): Add documentation for aligned_alloc
	and suggest it as an alternative to posix_memalign.
	(Hooks for Malloc): Document __memalign_hook is also called
	for aligned_alloc.  (Summary of Malloc): Add summary for
	aligned alloc.  Document __memalign_hook is also called
	for aligned_alloc.
Add missing bug number to NEWS.
Move tests of atan2, hypot and pow from libm-test.inc to auto-libm-test-in.