slang: use specific build and install commands for static builds

Doing this we ensure that only the static library and a static version
of slsh are built.

Fixes:
  http://autobuild.buildroot.net/results/f12/f124ca3737baf5aca32029226805133d3544715c/



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>