Default __MAKE_SHELL to /bin/sh when generating aton_ether_subr.c via

`gen_ether_subr`.

__MAKE_SHELL is only defined when installworld is run on stable/10,
which breaks workflows dealing with source trees mounted with noexec [*]

This is a direct commit to stable/10

Reported by: Mark Martinec <Mark.Martinec+freebsd@ijs.si>
Sponsored by: EMC / Isilon Storage Division

lib/libc/tests/net/Makefile

@@ -28,6 +28,8 @@ PROGS+=		h_dns_server
 DPADD.h_nsd_recurse+=	${LIBPTHREAD}
 LDADD.h_nsd_recurse+=	-lpthread
 
+__MAKE_SHELL?=	/bin/sh
+
 CLEANFILES+=	aton_ether_subr.c
 aton_ether_subr.c: gen_ether_subr ${.CURDIR:H:H:H:H}/sys/net/if_ethersubr.c
 	${__MAKE_SHELL} ${.ALLSRC} ${.TARGET}