s3:wscript: fix flex and bison detection message when not installed

s3:wscript: fix flex and bison detection message when not installed

If flex or bison are not installed, conf.env['BISON'] and
conf.env['FLEX'] respectively return an empty string, so
conf.CHECK_COMMAND() runs

 $ /bin/sh -c " --version  | head -n1"

and

 $ /bin/sh -c " --version"

which results in the following message

  /bin/sh: []: command not found

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Noel Power <npower@samba.org>