Link all required libraries to libisc

It would fail to link -lisc without additional libraries, which should
not be required.

lib/isc/Makefile.in

@@ -81,7 +81,7 @@ SRCS =		pk11.c pk11_result.c \
 		symtab.c task.c taskpool.c timer.c \
 		tm.c version.c
 
-LIBS =		${OPENSSL_LIBS} @LIBS@
+LIBS =		${OPENSSL_LIBS} ${JSON_C_LIBS} ${LIBXML2_LIBS} ${ZLIB_LIBS} @LIBS@
 
 # Note: the order of SUBDIRS is important.
 # Attempt to disable parallel processing.