assignment context to retrieve both the return value (if any) and the
C errno variable as an error (use _ to skip the result value if the
function returns void). For example:

	n, err = C.sqrt(-1)
	_, err := C.voidFunc()
	var n, err = C.sqrt(1)

Calling C function pointers is currently not supported, however you can
declare Go variables which hold C function pointers and pass them

pkg go/doc, func IsPredeclared(string) bool
pkg go/types, func Default(Type) Type
pkg go/types, func IdenticalIgnoreTags(Type, Type) bool
pkg math/big, method (*Float) Scan(fmt.ScanState, int32) error
pkg math/big, method (*Int) Sqrt(*Int) *Int
pkg math/rand, func Uint64() uint64
pkg math/rand, method (*Rand) Uint64() uint64
pkg math/rand, type Source64 interface, Int63() int64
pkg math/rand, type Source64 interface { Int63, Seed, Uint64 }

pkg math, func Erfcinv(float64) float64
pkg math, func Erfinv(float64) float64
pkg math, func Round(float64) float64
pkg math, func RoundToEven(float64) float64
pkg math/big, const MaxBase = 62
pkg math/big, method (*Float) Sqrt(*Float) *Float
pkg math/big, method (*Int) CmpAbs(*Int) int
pkg math/rand, func Shuffle(int, func(int, int))
pkg math/rand, method (*Rand) Shuffle(int, func(int, int))