tools: ocaml: In configure, check for ocamlopt

tools: ocaml: In configure, check for ocamlopt

If ocaml.m4 didn't find ocamlopt, disable all the ocaml builds.

Currently our Makefiles do not work properly when the native code
compiler (`ocamlopt') is not available.  In principle this should be
fixed to fall back to bytecode, but this is not a task for this stage
of the Xen 4.9 release.

Without this change, we cannot build on systems with only ocamlc.
That includes Debian jessie ARM64, as used on the new ARM64 hardware
in the Xen Project CI test lab.

When the Makefiles are fixed, this commit should be reverted.

Committers: Please rerun autogen.sh.

CC: Julien Grall <julien.grall@arm.com>
CC: Christian Lindig <christian.lindig@citrix.com>
CC: Jonathan Ludlam <Jonathan.Ludlam@citrix.com>
CC: David Scott <dave@recoil.org>
CC: Wei Liu <wei.liu2@citrix.com>
Tested-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>