Use correct curl-config program

Instead of calling again curl-config in src/Makefile (which ends up
calling the wrong curl-config: the one in the PATH instead of the one
pointed at by the environment variables at configure time), let's use
the CURL_LDADD variable which was computed at configure time.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/src/Makefile
===================================================================
--- a/src/Makefile
+++ b/src/Makefile
@@ -56,7 +56,7 @@
   TRANSPORT_MODS += blddir/lib/curl_transport/curltransaction
   TRANSPORT_MODS += blddir/lib/curl_transport/curlmulti
   TRANSPORT_MODS += blddir/lib/curl_transport/lock_pthread
-  TRANSPORT_LIBDEP += $(shell curl-config --libs)
+  TRANSPORT_LIBDEP += $(CURL_LDADD)
 endif
 ifeq ($(MUST_BUILD_LIBWWW_CLIENT),yes)
   TRANSPORT_MODS += blddir/lib/libwww_transport/xmlrpc_libwww_transport