Avoid corrupting diagnostic state with _Pragma changes.

The macros for ICC and MSVC correctly push and pop the diagnostic
state of the compiler when disabling deprecation warnings. The
ones for clang/gcc should do the same. Without this, if a blanket
deprecation warning is applied to the code base it'll be flipped
back on incorrectly with FF_ENABLE_DEPRECATION_WARNINGS.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
