execline: restore --shebangdir configure option

execline: restore --shebangdir configure option

Passing the option --shebangdir=/usr/bin to the configuration script adds the
CPP definition EXECLINE_SHEBANGPREFIX to
execline-x.y.z/src/include/execline/config.h. It is used by `s6-rc-compile` from
the s6-rc package to set the path to the execline interpreter in the scripts it
generates.

So, when building the host variant of execline, this path will be used in the
target service scripts generated by the host variant of `s6-rc-compile`. If not
forced to /usr/bin, the location of the execline interpreter on the target, it
will default to $(HOST_DIR)/bin thus leading to non-working scripts on the
target.

So, restore this option for the host variant.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>