From LWG2281:
There is a footnote at section 17.6.2.3 [using.linkage]/2 that reads:

"The only reliable way to declare an object or function signature from the Standard C library is by including the header that declares it, notwithstanding the latitude granted in 7.1.7 of the C Standard."

There is no section 7.1.7 in the C99 Standard (or C11 final draft). I think the relevant section is:

"Provided that a library function can be declared without reference to any type defined in a header, it is also permissible to declare the function and use it without including its associated header."

at 7.1.4/2 from C99.

Proposed wording:

The only reliable way to declare an object or function signature from the Standard C library is by including the header that declares it, notwithstanding the latitude granted in 7.1.77.1.4 of the C Standard.