Fix compatibility with GCC 6 and earlier

Fix compatibility with GCC 6 and earlier

fb0c44a fixed compilation on GCC 8 by using:
1) #pragma GCC diagnostic ignored "-Wstringop-truncation"
2) __attribute__ ((fallthrough));

However, -Wstringop-truncation has only been introduced in GCC 8 and
__attribute__ ((fallthrough)) has only been introduced in GCC 7.
Hence they caused Warnings/Errors on earlier versions.

Comments about fallthough (see https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/)
can be used on GCC 8 to prevent fall through warnings

Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>
Change-Id: I6f22e023191bbf22d8b01f1812061795a6494134