Fix the new warnings during autogen.sh

Follow-up fix to 3d6e2830876c for configure.in additions in master that
weren't in maint-0.2.2

configure.in

@@ -359,7 +359,7 @@ if test "$enable_bufferevents" = "yes" ; then
 
     # Check for the right version.  First see if version detection works.
     AC_MSG_CHECKING([whether we can detect the Libevent version])
-    AC_COMPILE_IFELSE([
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 #include <event2/event.h>
 #if !defined(LIBEVENT_VERSION_NUMBER) || LIBEVENT_VERSION_NUMBER < 10
 #error
@@ -367,13 +367,13 @@ int x = y(zz);
 #else
 int x = 1;
 #endif
-  ], [event_version_number_works=yes; AC_MSG_RESULT([yes]) ],
+  ])], [event_version_number_works=yes; AC_MSG_RESULT([yes]) ],
      [event_version_number_works=no;  AC_MSG_RESULT([no])])
     if test "$event_version_number_works" != 'yes'; then
       AC_MSG_WARN([Version detection on Libevent seems broken.  Your Libevent installation is probably screwed up or very old.])
     else
       AC_MSG_CHECKING([whether Libevent is new enough for bufferevents])
-      AC_COMPILE_IFELSE([
+      AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 #include <event2/event.h>
 #if !defined(LIBEVENT_VERSION_NUMBER) || LIBEVENT_VERSION_NUMBER < 0x02000700
 #error
@@ -381,7 +381,7 @@ int x = y(zz);
 #else
 int x = 1;
 #endif
-   ], [ AC_MSG_RESULT([yes]) ],
+   ])], [ AC_MSG_RESULT([yes]) ],
       [ AC_MSG_RESULT([no])
         AC_MSG_ERROR([Libevent does not seem new enough to support bufferevents.  We require 2.0.7-rc or later]) ] )
     fi