package/xmlstarlet: fix static linking

The static linking with libxml2 and lib(e)xslt (--enable-static-libs)
need to be enabled when BR2_PREFER_STATIC_LIB is set.

Fixes:
http://autobuild.buildroot.net/results/0fd/0fd4a65ab98a4cf3fe893c29628608a71e96ecbc/build-end.log

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/xmlstarlet/xmlstarlet.mk

@@ -12,9 +12,15 @@ XMLSTARLET_LICENSE_FILES = COPYING
 XMLSTARLET_DEPENDENCIES += libxml2 libxslt \
 	$(if $(BR2_PACKAGE_LIBICONV),libiconv)
 
-XMLSTARLET_CONF_OPT += --disable-static-libs \
+XMLSTARLET_CONF_OPT += \
 	--with-libxml-prefix=${STAGING_DIR}/usr \
 	--with-libxslt-prefix=${STAGING_DIR}/usr \
 	--with-libiconv-prefix=${STAGING_DIR}/usr
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+XMLSTARLET_CONF_OPT += --enable-static-libs
+else
+XMLSTARLET_CONF_OPT += --disable-static-libs
+endif
+
 $(eval $(autotools-package))