Old versions of gcc give warnings for pragma diagnostic push and pop

[teeks99]

#pragma GCC diagnostic push and #pragma GCC diagnostic pop are only supported in gcc >= 4.6.

A solution to this would be to change the guards from

#ifdef __GNUC__
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wstrict-aliasing"
#endif  // __GNUC__

and

#ifdef __GNUC__
  #pragma GCC diagnostic pop
#endif  // __GNUC__

To:

#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wstrict-aliasing"
#endif  // __GNUC__

and

#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
  #pragma GCC diagnostic pop
#endif  // __GNUC__

There are approximately a dozen header files that do this, as well as the cpp_file.cc codegen. This could be applied to the 3.?.x branches, but might not be needed on master....if master is now c++11, gcc versions before 4.6 might not work with it anyway...I guess it depends on how good various gcc's -std=c++0x support was back then. Might not hurt to apply to master as well.

[xfxyjwf]

@teeks99 We no longer support old gcc versions on the master branch, but we will keep the 3.5.x branch for pre-C++11 support.

[mcy]

GCC versions older than 6.3 are not supported. See https://opensource.google/documentation/policies/cplusplus-support#2_compilers