configure & compiler warning cleanup

• configure: Use LT_INIT from libtool 2 instead of deprecated AC_PROG_LIBTOOL
• Quiet -Wredundant-decls from xorg/os.h fallbacks for new libc functions
• Fix 24 of 24 -Wimplicit-fallthrough warnings
• Fix -Wanalyzer-possible-null-argument warning in ATIDRIScreenInit