tools/ocaml: use APPEND_CFLAGS and APPEND_LDFLAGS

tools/ocaml: use APPEND_CFLAGS and APPEND_LDFLAGS

While implementing the --enable-rpath change I noticed that a ocaml
build does not use APPEND_LDFLAGS. Make use of APPEND_CFLAGS and
APPEND_LDFLAGS as it is done already in other places.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: David Scott <dave.scott@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>