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>

package/slang/slang.mk

@@ -48,4 +48,10 @@ ifeq ($(BR2_PACKAGE_READLINE),y)
 	SLANG_DEPENDENCIES += readline
 endif
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+	SLANG_MAKE_OPTS = static
+	SLANG_INSTALL_STAGING_OPTS = DESTDIR=$(STAGING_DIR) install-static
+	SLANG_INSTALL_TARGET_OPTS = DESTDIR=$(TARGET_DIR) install-static
+endif
+
 $(eval $(autotools-package))