The Makefiles seems to use LADD as LDFLAGS variable, but just setting it does not seem to be enough, a oneliner might be needed.
Notes to myself: Setting LADD works but leads to more --as-needed issues, e. g.: x86_64-pc-linux-gnu-gcc -o auth_client -Wl,-O1 -Wl,--as-needed auth_client.o -L/usr/lib64 -lcurl /var/tmp/portage/dev-libs/xmlrpc-c-1.06.09-r1/work/xmlrpc-c-1.06.09/src/.libs/libxmlrpc_client.a /var/tmp/portage/dev-libs/xmlrpc-c-1.06.09-r1/work/xmlrpc-c-1.06.09/src/.libs/libxmlrpc.a /var/tmp/portage/dev-libs/xmlrpc-c-1.06.09-r1/work/xmlrpc-c-1.06.09/lib/libutil/.libs/libxmlrpc_util.a -lxml2 -lz -lm /var/tmp/portage/dev-libs/xmlrpc-c-1.06.09-r1/work/xmlrpc-c-1.06.09/src/.libs/libxmlrpc_client.a(xmlrpc_curl_transport.o): In function `curlMulti_perform': /var/tmp/portage/dev-libs/xmlrpc-c-1.06.09-r1/work/xmlrpc-c-1.06.09/lib/curl_transport/xmlrpc_curl_transport.c:290: undefined reference to `curl_multi_perform'
Fixed in CVS.