build: fix bootstrap, set language to C++

Bootstrap script doesn't start with #! line.
Set the language in the build to C++.

Change-Id: I171dac4550063655b11d88af7009f5f6d129a34f
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 5c033b0..12e6b0f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -7,6 +7,7 @@
 AC_CONFIG_HEADERS([config.h])
 AM_INIT_AUTOMAKE([subdir-objects -Wall -Werror foreign dist-xz])
 AM_SILENT_RULES([yes])
+AC_LANG([C++])
 
 # Checks for programs.
 AC_PROG_CXX