attribute: const/pure defaults to unsequenced/reproducible
Also, update the commentary to reflect the fact that unsequenced
and reproducible functions can inspect and update storage
addressed by their arguments, and clarify the business about
returning once, and about REPRODUCIBLE and UNSEQUENCED applying to
pointers to functions and to function types. It’s not clear that
we should encourage the use of REPRODUCIBLE and UNSEQUENCED yet,
as the support is not there and the likely benefits are small.
* m4/gnulib-common.m4 (_GL_ATTRIBUTE_CONST): Default to
_GL_ATTRIBUTE_UNSEQUENCED if the const attribute is lacking,
as every const function is also unsequenced.
(_GL_ATTRIBUTE_PURE): Similarly, default to _GL_ATTRIBUTE_REPRODUCIBLE
as every pure function is reproducible.