some versions that support those options don't support disabling yyunput
by defining YY_NO_UNPUT.
If it doesn't support those options, don't check if it supports
generating reentrant scanners, as we can't add any --noFUNCTION options
to suppress generation of functions.
for -o and Flex's support for it in a way that lets us more easily fail
if Lex/Flex fails (so that we don't try to compile a bogus scanner.c
that might be generated; that appears to have happened on at least one
occasion, with the resulting scanner.o missing some functions, causing
weird errors in configure scripts for programs using libpcap), and also
prepares us to handle newer versions of Flex where we want Flex to
generate a header file so we don't get "defined but not declared"