In that case I'd rather change the module's description to "covers whatever the linked C run-time library provides, exposed as C99 interface" ;), since imho core.stdc.math should only provide a binding. The proposed version goes a bit further by implementing C macros, providing the "long double" wrappers and renaming functions to conform to C99 as much as possible.
I'd suggest making Phobos' std.math more core.stdc.math-independent instead.