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 warning: redundant redeclaration of ‘timingsafe_memcmp’
• Fix prototype warnings for CG6HWCursorInit()
• Fix warning: variable ‘pCg6’ set but not used [-Wunused-but-set-variable]