Make libdvdread compile under QNX

Here is a really small patch which fixes compilation under QNX.

Patch provided by Mike Gorchak
<mike DOT gorchak DOT qnx AT gmail DOT com>. Thanks!
Add ax_check_compile_flag to the m4 files

A new GCC compiler flag has made it so that it would be convenient to
check GCC compiler flags.
Fix packed structures with mingw gcc >= 4.7

As of gcc 4.7, structures are packed in a "MS compatible" format by
default.  If you want the correct behavior, you must add
-mno-ms-bitfields to CFLAGS :-\

Original patch provided by John Stebbins
<stebbins AT jetheaddev DOT com>. This patch implemented an idea
that was put forward by Mike Frysinger <vapier AT gentoo DOT org>.

Thanks to both!