configure.ng: use AM_PROG_AR to check ar(1) command

This fixes

  automake-1.12/am/library.am: warning: 'libngipaddr.a':
   linking libraries using a non-POSIX .../automake-1.12/am/library.am:
   archiver requires 'AM_PROG_AR' in 'configure.in'
   src/ipaddr/Makefile.am:12: while processing library 'libngipaddr.a'

and similar warnings of automake.

.gitignore

@@ -4,6 +4,7 @@ aclocal.m4
 ansi2knr.1
 ansi2knr.c
 ansi2knr.h
+ar-lib
 autom4te.cache
 build-stamp-ngircd*
 config.log

Makefile.am

@@ -22,7 +22,7 @@ clean-local:
 maintainer-clean-local:
 	rm -rf autom4te.cache
 	rm -f Makefile.in Makefile aclocal.m4 configure
-	rm -f mkinstalldirs missing depcomp install-sh
+	rm -f ar-lib mkinstalldirs missing depcomp install-sh
 	rm -f config.log debian
 
 testsuite:

configure.ng

@@ -60,6 +60,7 @@ AC_PROG_CC_STDC
 
 # -- Helper programs --
 
+AM_PROG_AR
 AC_PROG_AWK
 AC_PROG_INSTALL
 AC_PROG_LN_S