As these options are 'off-by-default' and don't have anything in the 'else' clause, can we abbreviate them -- move the CFLAGS into the AC_ARG_ENABLE?  ```  AC_ARG_ENABLE([fsanitize-ubsan],      [AS_HELP_STRING([--enable-fsanitize-ubsan], [Turn on Undefined Behavior Sanitizer (for developers)])],      [CFLAGS="$CFLAGS -fsanitize=undefined -fno-sanitize-recover=undefined"])  ```