Also: set the build and install directories to the source directory
because the build system unfortunately expects the `src_ext` directory
to be under the current working directory when building the bundled
third-party libraries, even when the configure script is run from
another directory.
@scemama pointed out that 'make' just calls 'dune' which is already
parallel, so make itself should not have more than one job.
opam@:2.1 need 'make lib-ext' for cmdliner, above it's obsolete.
Co-authored-by: Bernhard Kaindl <bernhard.kaindl@cloud.com>
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