The NetBSD C library's headers do not expose extensions when
compiling with -std=c99 (as opposed to -std=gnu99 or no -std=
option), so define _NETBSD_SOURCE to get timegm, and functions
that would otherwise require an _XOPEN_SOURCE definition, e.g.
realpath.
Note that, as with FreeBSD, you need gmake to compile janet
on NetBSD, and can also install it from packages.
This should help address leaking file descriptors in multithreaded
programs. There are a few cases where a race can occur though, as
some apis (fopen and mktemp).
Because we use an amalgated build, feature
test macros should be set in a single file that
is included before any other headers, and is placed
at the top of the amalgamated build.
