check that the go stdlib was built with the required options
AFAICT, go build tags are a compile-time only construct, so there is
no way to query whether a certain package was built with a specific
tag. However, the 'netgo' tag triggers conditional compilation of the
cgo_stubs.go file, and that file name ends up in the executable. (we
could have looked for specific symbols in the 'net.a' library instead,
but that requires locating it first, which is more hassle than it is
worth)

issue #93