Accept relative paths to AR as well.

https://bugs.gentoo.org/720966
--- a/configure.in
+++ b/configure.in
@@ -105,7 +105,7 @@ AC_LIBTOOL_WIN32_DLL
 AC_PROG_LIBTOOL
 AC_PROG_INSTALL
 AC_PROG_LN_S
-AC_PATH_PROG(AR, ar, no)
+AC_CHECK_PROG(AR, ar, no)
 if [[ $AR = "no" ]] ; then
     AC_MSG_ERROR("Could not find ar - needed to create a library")
 fi