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!

configure.ac

@@ -19,6 +19,7 @@ m4_define([dvdread_pre],   [])
 
 AC_INIT([libdvdread], [dvdread_major.dvdread_minor.dvdread_sub[]dvdread_pre])
 AC_CONFIG_SRCDIR([src/dvdread/dvd_reader.h])
+m4_include([m4/ax_check_compile_flag.m4])
 AM_INIT_AUTOMAKE([1.6])
 
 dnl create a config.h file (Automake will add -DHAVE_CONFIG_H)
@@ -202,6 +203,11 @@ DEBUG_CFLAGS="-g -DDEBUG $CFLAGS"
 
 AC_SUBST(DEBUG_CFLAGS)
 
+dnl ---------------------------------------------
+dnl Check for bitfield compiler flag
+dnl ---------------------------------------------
+AX_CHECK_COMPILE_FLAG([-mno-ms-bitfields], [CFLAGS="-mno-ms-bitfields $CFLAGS"])
+
 dnl ---------------------------------------------
 dnl Check for doxygen (dynamic documentation generator)
 dnl ---------------------------------------------