build: set language to C++

Set the language in the build to C++.

Change-Id: Id95ca63533ea6536f4dfd5b5b2f0be7634482948
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 3e29f20..66ee856 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