CVS tadam: Use sed found in $PATH, not hard-coded to /bin/sed

[cvs]

CVSROOT:        /home/cvs/fvwm
Module name:    fvwm
Changes by:     tadam   11/11/07 13:36:16

Modified files:
        .              : Tag: branch-2_6 ChangeLog configure.ac 
        bin            : Tag: branch-2_6 ChangeLog fvwm-perllib.in 

Log message:
Use sed found in $PATH, not hard-coded to /bin/sed

We already have a check for sed when compiling the HTML documentation, but
never used it for more than this; fvwm-perllib will benefit from such a
check.

We don't bother doing this for fvwm-menu-* though.

Diff from Dominik Vogt.  Tweaked by tadam@ slightly.