automake: Add AM_PROG_AR before LT_INIT to silence a lot of warnings.

The warnings appear to occur with newer automake (probably 1.12).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/configure.ac b/configure.ac
index 88a2df6..29ee87b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,6 +21,8 @@
 # to make
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
+
 LT_PREREQ([2.2])
 LT_INIT([disable-static])