configure: bump libdrm version requirement to 2.4.23

NOTE: This is a candidate for the 7.10 (and 7.9?) branch.
diff --git a/configure.ac b/configure.ac
index 9cbced5..b6f7d86 100644
--- a/configure.ac
+++ b/configure.ac
@@ -18,11 +18,11 @@
 AC_CANONICAL_HOST
 
 dnl Versions for external dependencies
-LIBDRM_REQUIRED=2.4.15
-LIBDRM_RADEON_REQUIRED=2.4.17
+LIBDRM_REQUIRED=2.4.23
+LIBDRM_RADEON_REQUIRED=2.4.23
 DRI2PROTO_REQUIRED=2.1
 GLPROTO_REQUIRED=1.4.11
-LIBDRM_XORG_REQUIRED=2.4.17
+LIBDRM_XORG_REQUIRED=2.4.23
 LIBKMS_XORG_REQUIRED=1.0.0
 
 dnl Check for progs