Fix build on NetBSD.

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.

README.md

@@ -115,6 +115,11 @@ gmake test
 gmake repl
 ```
 
+### NetBSD
+
+NetBSD build instructions are the same as the FreeBSD build instuctions.
+Alternatively, install directly from packages, using `pkgin install janet`.
+
 ### Windows
 
 1. Install [Visual Studio](https://visualstudio.microsoft.com/thank-you-downloading-visual-studio/?sku=Community&rel=15#) or [Visual Studio Build Tools](https://visualstudio.microsoft.com/thank-you-downloading-visual-studio/?sku=BuildTools&rel=15#)

src/core/features.h

@@ -38,4 +38,11 @@
 #define _XOPEN_SOURCE 500
 #endif
 
+/* Needed for timegm and other extensions when building with -std=c99.
+ * It also defines realpath, etc, which would normally require
+ * _XOPEN_SOURCE >= 500. */
+#if !defined(_NETBSD_SOURCE) && defined(__NetBSD__)
+#define _NETBSD_SOURCE
+#endif
+
 #endif