diag-control.h: define PCAP_DO_PRAGMA() iff we're going to use it.

diag-control.h: define PCAP_DO_PRAGMA() iff we're going to use it.

Test for whether we're using Clang 2.8 or later or GCC 4.6 or later,
rather than testing for MSVC; that way, we define it for Microsoft's
clang-cl (which is Clang-based, so we want it defined, but it defines
_MSC_VER) and don't define it for compilers that aren't MSVC, Clang, or
GCC.

This should fix issue #1115.

(cherry picked from commit e985d6564ce124c9b85084623646bd0210f47d42)