Only run native tests if ocamlopt is available

[infinity0]

Otherwise the tests fail on architectures without ocamlopt, e.g. https://buildd.debian.org/status/fetch.php?pkg=ocamlbuild&arch=mips64el&ver=0.11.0-1&stamp=1501347472&raw=0

Reading Makefile.config to check ocamlopt existence, is not the most elegant; I'd be happy to change if there's a better way.

[whitequark]

This can all be replaced with line = "OCAML_NATIVE=true", can't it?

[gasche]

Another way to do this, which I personally think would be nicer, would be to call #mod_use "../src/ocamlbuil_config.ml";; to get configuration values.
But then there is the issue that OCAML_NATIVE is not currently exported in this config file, so you would need to add a line for this in configure.make. I think that having two extra declarations, ocaml_native and ocaml_native_tools, would be fine -- long-term it would be nice to reorder ocamlbuild_config to follow the more regular Makefile.config structure, but that is for another PR.

[infinity0]

Ah yes that is nicer, I did not notice src/ocamlbuild_config.ml before. I've updated the commit.

[gasche]

Very nice, thank you. Before I merge, would you like to add a change entry in the Changes file for the next release? I think the credit line "(Ximin Luo, review by whitequark and Gabriel Scherer)" would be fine.

[infinity0]

Changes updated.

[gasche]

Thanks! Merged.