do NOT force definition of _POSIX_SOURCE, at least not on NetBSD,

especially when you follow using non-POSIX vfork()

defining _POSIX_SOURCE means "POSIX clean environment", which means
(besides other things) that we won't get proper definition of e.g.
vfork() and end up with the compatibility stub

gnu/dist/toolchain/libio/libioP.h

@@ -23,9 +23,12 @@
    other reasons why the executable file might be covered by the GNU
    General Public License.  */
 
+#ifndef __NetBSD__
+/* the forced define of _POSIX_SOURCE is probably only needed for glibc */ 
 #ifndef _POSIX_SOURCE
 # define _POSIX_SOURCE
 #endif
+#endif
 
 #include <errno.h>
 #ifndef __set_errno