Avoid g++ "warning: attributes are not permitted in this position".
* m4/gnulib-common.m4 (gl_COMMON_BODY): Add more comments. Define
_GL_BRACKET_USABLE. Use it for _GL_ATTRIBUTE_DEPRECATED,
_GL_ATTRIBUTE_MAYBE_UNUSED, _GL_ATTRIBUTE_NODISCARD,
_GL_ATTRIBUTE_REPRODUCIBLE, _GL_ATTRIBUTE_UNSEQUENCED.
* lib/stdio.in.h (aszprintf, vaszprintf, asprintf, vasprintf): Move
_GL_ATTRIBUTE_NODISCARD to the same position as the other attributes.
* lib/c-vasprintf.h (c_aszprintf, c_vaszprintf, c_asprintf,
c_vasprintf): Likewise.