Emacs can use this macro, so make it public.
* doc/intprops.texi (Arithmetic Type Properties): Rename from
'Integer Type Determination', since some of these macros apply
to non-integer types. Clarify what kinds of constant expressions
these macros return. Say when the arguments can be non-integers.
Mention newly published macro EXPR_SIGNED.
* lib/intprops.h (EXPR_SIGNED): Rename from _GL_INT_SIGNED, to
make it public. All uses changed.