libzds: add curl interface to access zosmf rest api

Add library functions that allow to communicate with z/OSMF REST services
using libcurl.
Following three functions are added:

lzds_rest_get_enq()
	to obtain an ENQ that will mark a dataset as in use to z/OS until
	it is released again
lzds_rest_release_enq()
	to release an ENQ and mark a dataset as no longer in use to z/OS
lzds_rest_ping()
	to ping a z/OSMF REST server to check if it accessible
	or to refresh an ENQ and prevent it from a timeout after 10 minutes

Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

libzds/Makefile

@@ -2,6 +2,10 @@ include ../common.mak
 
 ALL_CFLAGS += -D_FILE_OFFSET_BITS=64
 
+ifneq (${HAVE_CURL},0)
+ALL_CFLAGS += -DHAVE_CURL
+endif
+
 lib = libzds.a
 
 all: $(lib)