Accept glibc 2.20 as valid.

The pre-releases were all called 2.19.90 and worked fine.
But now that glibc 2.20 final is out configure wouldn't recognize it.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14505 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/configure.ac b/configure.ac
index e2ebcfe..336445c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1008,6 +1008,13 @@
 	DEFAULT_SUPP="glibc-2.34567-NPTL-helgrind.supp ${DEFAULT_SUPP}"
 	DEFAULT_SUPP="glibc-2.X-drd.supp ${DEFAULT_SUPP}"
 	;;
+     2.20)
+	AC_MSG_RESULT(2.20 family)
+	AC_DEFINE([GLIBC_2_20], 1, [Define to 1 if you're using glibc 2.20.x])
+	DEFAULT_SUPP="glibc-2.X.supp ${DEFAULT_SUPP}"
+	DEFAULT_SUPP="glibc-2.34567-NPTL-helgrind.supp ${DEFAULT_SUPP}"
+	DEFAULT_SUPP="glibc-2.X-drd.supp ${DEFAULT_SUPP}"
+	;;
      darwin)
 	AC_MSG_RESULT(Darwin)
 	AC_DEFINE([DARWIN_LIBC], 1, [Define to 1 if you're using Darwin])