How to get isblank?

[Reuben Thomas]

As far as I can see, there is an implementation of isblank in gnulib
for systems that lack it, but I can't see how I'm supposed to use it:
I can see the module c-ctypes which seems to be specifically for the C
locale, and unictype/ctype-blank module, which mentions
"generalisation", though it's not clear to me of what. What do I use
if I just want to get the C99 "isblank" macro/function from ctype.h?

Sorry if I'm being dense, but perhaps there's an opportunity for
clarification here?

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