wafsamba/samba_waf18: redefine flex function

wafsamba/samba_waf18: redefine flex function

There is a bug in waf: flex routine adjusts its inputs against
the task's current working directory but assumes it is being called from
within the build variant directory.

For Samba this means we adjust one level up than the actual work
directory we use to run (bin/ vs bin/default) and flex doesn't find the
source files.

Fix the issue by creating a local override of flex definition that
utilizes the same workd directory for both path adjustment and running
the flex itself.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri Sep  7 15:45:36 CEST 2018 on sn-devel-144