change configure.ac to use c++ version
diff --git a/configure.ac b/configure.ac
index 3f23767..7cf5200 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,7 +17,7 @@
# In the SVN trunk, the version should always be the next anticipated release
# version with the "-pre" suffix. (We used to use "-SNAPSHOT" but this pushed
# the size of one file name in the dist tarfile over the 99-char limit.)
-AC_INIT([Protocol Buffers],[21.0-rc-1],[protobuf@googlegroups.com],[protobuf])
+AC_INIT([Protocol Buffers],[3.21.0-rc-1],[protobuf@googlegroups.com],[protobuf])
AM_MAINTAINER_MODE([enable])
