pub fn c_library_available(soname: text) -> boolean env#readReturns whether the C library soname is usable right now: it loads, and every #c binding declared against it resolves in it.
Ask this before calling into a library declared with [c] optional-libs, which is not installed on every machine and is loaded only when a binding needs it. Both halves of the answer matter — a library of the wrong version loads and then exports only some of the symbols, so "the file is there" would say yes where the call still fails.
A library the program never declared answers false.