Yp [glibc 2.26]: Support using libtirpc.

Glibc 2.26 has removed the Sun RPC implementation, so try using
libtirpc if it exists.

.gitattributes

@@ -609,7 +609,6 @@ testfont binary
 /src/modules/SANE/sane.c foreign_ident
 /src/modules/Yp/Makefile.in foreign_ident
 /src/modules/Yp/acconfig.h foreign_ident
-/src/modules/Yp/configure.in foreign_ident
 /src/modules/Yp/module.pmod.in foreign_ident
 /src/modules/Yp/yp.c foreign_ident
 /src/modules/_Charset/Makefile.in foreign_ident

src/modules/Yp/configure.in

-# $Id: configure.in,v 1.13 2005/06/16 20:51:56 grubba Exp $
+# $$
 AC_INIT(yp.c)
 AC_CONFIG_HEADER(config.h)
 
@@ -6,6 +6,9 @@ AC_MODULE_INIT()
 
 AC_CHECK_LIB(nsl, gethostbyname)
 
+# Glibc 2.26 and later don't include the Sun RPC implementation anymore.
+PIKE_PKG_CONFIG(libtirpc)
+
 AC_HAVE_HEADERS(sys/types.h rpc/types.h rpc/rpc.h rpc/clnt.h \
 		rpcsvc/ypclnt.h rpcsvc/yp_prot.h,,,[
 #ifdef HAVE_RPC_TYPES_H