Tools like lcov (for code coverage) does not like files named
"<stdout>". For example it reports errors like:
genhtml: ERROR: cannot read
/usr/src/selinux/libsemanage/src/<stdout>
When using flex -o option, the output file name gets written in the
generated C code, which solves this issue.
Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>