guile: fix libltdl, libgmp and libunistring detection

Explicitly pass the path to libtldl, libgmp and libunistring, so that
guile does not try to use host versions when available.

Fixes:

  http://autobuild.buildroot.org/results/056/056b7fcbf3b73a8d15ca635b000771cdc2d7fbf9/

And another similar problem with libgmp and libunistring.

[Thomas: added --with-libunistring-prefix, as suggested by Samuel
Martin.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>

package/guile/guile.mk

@@ -29,5 +29,10 @@ GUILE_CFLAGS = \
 GUILE_CONF_ENV += GUILE_FOR_BUILD=$(HOST_DIR)/usr/bin/guile \
 	CFLAGS="$(TARGET_CFLAGS) $(GUILE_CFLAGS)"
 
+GUILE_CONF_OPTS += \
+	--with-libltdl-prefix=$(STAGING_DIR)/usr/lib \
+	--with-libgmp-prefix=$(STAGING_DIR)/usr/lib \
+	--with-libunistring-prefix=$(STAGING_DIR)/usr/lib
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))