stdlib: port to Solaris 2.6

stdlib: port to Solaris 2.6

Also, the code worked on Solaris 7 through 9 only by accident.
Problem reported by Tom G. Christensen in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-01/msg00059.html>.
* lib/stdlib.in.h: If __need_system_stdlib_h is defined,
simply include the system stdlib.h.
* lib/getopt.in.h (__need_system_stdlib_h):
* lib/pthread.in.h (__need_system_stdlib_h):
* lib/unistd.in.h (__need_system_stdlib_h) [!__GLIBC__]:
Define when including <stdlib.h>, to avoid problems at least for
the pthread case on Solaris 2.6 and 7.  These .h files can get by
with the system stdlib.h.