configure.ac: require protobuf >= 2.5.0

In order to add compatibility with the latest protobuf 2.6.0 release, we
need to add an option to the t/test-full.proto schema that was
introduced in protobuf 2.5.0, so we need to depend on protobuf 2.5.0 or
higher now.

configure.ac

@@ -47,7 +47,7 @@ AC_ARG_ENABLE([protoc],
   AS_HELP_STRING([--disable-protoc], [Disable building protoc_c (also disables tests)]))
 if test "x$enable_protoc" != "xno"; then
   AC_LANG_PUSH([C++])
-  PKG_CHECK_MODULES([protobuf], [protobuf])
+  PKG_CHECK_MODULES([protobuf], [protobuf >= 2.5.0])
 
   save_CPPFLAGS="$CPPFLAGS"
   CPPFLAGS="$save_CPPFLAGS $protobuf_CFLAGS"