slang: add adapted upstream patches for static builds

These are adapted upstream patches to make them apply on the version we
are using in Buildroot, and also taking into account that we already
have another patch which modifies the same file, so these patches would
not apply as they are. Unnecessary parts of the upstream patches have
been ingnored, such as changelogs or version changes.
The purpose of these patches are allowing the static library and a
static version of slsh be built without building any dynamic bit.
These patches will be included in the next release, so we can remove
them when we do the next version bump.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Markos Chandras <Markos.Chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>