sysdeps/posix/tempname.c: Merge from gnulib

sysdeps/posix/tempname.c: Merge from gnulib

Partial merge from gnulib which fixes a number of -Wundef warnings.
The parts that differ from gnulib are the header comment, use of
__glibc_unlikely, a #define of __secure_getenv and the use of tabs.
The majority of the patch is cosmetic comment changes, the only runtime
change is an abort if an unknown kind is passed to __gen_tempname.

ChangeLog:

2014-06-25  Will Newton  <will.newton@linaro.org>

	* sysdeps/posix/tempname.c: Merge from gnulib, cosmetic
	comment changes throughout the file.  Remove checks
	for HAVE_*_H definitions that are not required.
	(__gen_tempname): Call abort if an unknown kind value is
	passed.