configure & compiler warning cleanups

• 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 20 out of 23 -Wshadow warnings
• Fix 2 of 2 -Wformat warnings
• Fix 40 of 42 -Wformat-nonliteral warnings
• Fix warning: no previous prototype for ‘sis_get_device’ [-Wmissing-prototypes]
• Fix 5 of 5 -Wstringop-truncation warnings