stdlib: support non-GCC __attribute__

stdlib: support non-GCC __attribute__

Fix a serious and tricky problem encountered when attempting to
add the getloadavg module to Emacs.  Emacs worked fine on RHEL
5.5, but it crashed due to memory corruption on Solaris 10 with
Sun C 5.11.  Emacs normally ORs 3-bit tags into their low-order
bits that are otherwise zero.  This tagging is optional inside
Emacs but is preferred and is used when __attribute__ ((__aligned
(8))) works, as it does with both recent-enough GCC and with Sun C
5.11.  However, Sun C 5.11 is not GCC and does not #define
__GNUC__ and __GNUC_MINOR__.

When I added the getloadavg module to Emacs, it brought in
stdlib.in.h, which contained this fragment:

   #ifndef __attribute__
   # if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 8)
   #  define __attribute__(Spec)   /* empty */
   # endif
   #endif

When files that include <stdlib.h> were compiled with Sun C 5.11,
the above code disabled __attribute__ ((__aligned (8))), which
caused variables to not be properly aligned, which eventually led
to the pointer corruption mentioned above.  (This was a bit hard
to diagnose, unfortunately.)

Several "#define __attribute__(X) /* empty */" code snippets need
to be eradicated from Gnulib to work with non-GCC compilers that
support __attribute__.  The Autoconf way to do this is to test for
each kind of attribute that we want support for, and selectively
enable that in source code.

Fix this problem just for stdlib.h, by adding a test for the
__noreturn__ attribute, and change stdlib.in.h to use that test
when needed.  This technique can be easily generalized to the
other *.in.h files and attributes, and a similar technique can be
used for *.h and *.c files.  This patch is enough to solve the
problem for Emacs + getloadavg, and I thought I'd publish it for
feedback before undertaking further, similar fixes in other
modules.

This patch does not arrange to #define HAVE_ATTRIBUTE_NORETURN
because it's not needed for stdlib.h.  It merely substitutes the
value directly into stdlib.h.  We may well need to #define it, or
similar symbols, for other modules, but it's nice to also have an
option to not #define it for applications like Emacs that do not
need it.

* lib/stdlib.in.h (__attribute__): Do not #define.
(_GL_ATTRIBUTE_NORETURN): New macro, which in stdlib.h needs to
be defined only if the _Exit module is also used.
* m4/_Exit.m4 (gl_FUNC__EXIT): Require gl_ATTRIBUTE_NORETURN.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Subst
HAVE_ATTRIBUTE_NORETURN and default it to 1, its value on GNU
platforms.
* modules/_Exit (Files): Add m4/attribute.m4.
* modules/stdlib (Makefile.am): Substitute HAVE_ATTRIBUTE_NORETURN.
* m4/attribute.m4: New file.