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 -Wmissing-prototypes & -Wnested-externs warning for TCXHWCursorInit()
• Fix warning: variable ‘pTcx’ set but not used [-Wunused-but-set-variable]