configure & compiler warnings 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
• Fix warning: variable ‘pCg14’ set but not used [-Wunused-but-set-variable]
• Quiet warning: redundant redeclaration of ‘timingsafe_memcmp’ [-Wredundant-decls]