Code should compile with --std=c++03 before we switch to c++11 only.

[liujisi]

I tried to compile a c++03 protoc on mac, but it seems to be broken: following is an incomplete list of the failures. We should test the c++03 compatibility before making the C++11 only switch.

1. va_copy undefined - used in src/google/protobuf/stubs/stringprintf.cc
   va_copy is only defined after C++11, however clang implicitly defines it when --std is not explicitly set. If --std=c++03 is passed, the function is not found. (need to use __va_copy)

2. isinf undefined - used in src/google/protobuf/stubs/mathlimits.h
   there's a hack to workaround the including issue for gcc4.9 bug (see comments in the mathlimits.h). But plain isinf is not defined for clang for non C++11.

[xfxyjwf]

I don't think our code base ever compiles with --std=c++03. The va_copy code has been there since always.