Don't interfere with a program's definition of __attribute__.

Don't interfere with a program's definition of __attribute__.

* lib/stdio.in.h (__attribute__): Remove definition.
(_GL_ATTRIBUTE_FORMAT, _GL_ATTRIBUTE_FORMAT_PRINTF): New macros.
(dprintf, fprintf, obstack_printf, obstack_printf, obstack_vprintf,
printf, snprintf, sprintf, asprintf, vasprintf, vdprintf, vfprintf,
vsnprintf, vsprintf): Use _GL_ATTRIBUTE_FORMAT_PRINTF.
* lib/string.in.h (__attribute__): Remove definition.
Reported by Paul Eggert.
(cherry picked from commit e680a55c761b3e7f9a3b9e62ac1a9336f83fb628)