Add an option to disable NIS

NIS is not necessarily available in uClibc, so we need an option to
not compile support for it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

---
 configure.ac |    6 ++++++
 1 file changed, 6 insertions(+)

Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -2698,6 +2698,12 @@
 		DISABLED_EXTENSIONS="${DISABLED_EXTENSIONS} _codecs_kr _codecs_jp _codecs_cn _codecs_tw _codecs_hk _codecs_iso2022"
 	fi])
 
+AC_ARG_ENABLE(nis,
+	AS_HELP_STRING([--disable-nis], [disable NIS]),
+	[ if test "$enableval" = "no"; then
+    	     DISABLED_EXTENSIONS="${DISABLED_EXTENSIONS} nis"
+  	  fi])
+
 AC_SUBST(TK)
 AC_ARG_ENABLE(tk,
 	AS_HELP_STRING([--disable-tk], [disable tk]),