build: set language to C++

Set the language in the build to C++.

Change-Id: Ib580d7470be92b198e2ad98ca4b7672aa5a3d26e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 9becfe4..07521ba 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3,6 +3,7 @@
 
 AM_INIT_AUTOMAKE([subdir-objects -Wall -Werror foreign dist-xz])
 AM_SILENT_RULES([yes])
+AC_LANG([C++])
 
 # Checks for programs
 AC_PROG_CXX