configure & compiler warning cleanup

• configure: Use LT_INIT from libtool 2 instead of deprecated AC_PROG_LIBTOOL
• Add X.Org's standard C warning flags to AM_CFLAGS
• Quiet -Wredundant-decls from xorg/os.h fallbacks for new libc functions
• Fix error: implicit declaration of function 'xf86UnMapVidMem'