Modify obstack.[hc] to avoid having to include other gnulib files
Using the standard gnulib obstack source requires importing quite a
lot of other files from gnulib, and requires build changes.
include/
* obstack.h (__attribute_pure__): Expand _GL_ATTRIBUTE_PURE.
libiberty/
* obstack.c (__alignof__): Expand alignof_type from alignof.h.
(obstack_exit_failure): Don't use exitfail.h.
(_): Include libintl.h when HAVE_LIBINTL_H and nls enabled.
Provide default. Don't include gettext.h.
(_Noreturn): Define.
* obstacks.texi: Adjust node references to external libc info files.