Add --bootstrap option

[eugpermar]

If configure script is called with --bootstrap option, and proper
instructions are added to build dep, it will be automatically downloaded
& built under vendor folder.

For example, to build a GIT revision of librd:

LIBRD_GIT_VERSION=73fae1cc0ca378669fa09bcda54b408397f80c18
function make_librd {
  if [[ -d "librd-$LIBRD_GIT_VERSION" && ! -d librd ]]; then
    mv "librd-$LIBRD_GIT_VERSION" librd
  fi

  cd librd
  make libs
}

And in checks function:

...
mkl_lib_check --static=-lrd \
      --bootstrap_url="https://github.com/edenhill/librd/archive/$LIBRD_GIT_VERSION.zip" \
      --bootstrap_makefn=make_librd \
      --bootstrap_cc_flags="-I./vendor/librd" \
      --bootstrap_static_lib=vendor/librd/librd/librd.a \
        "librd" "" fail CC "-lrd -lpthread -lrt" \
        "#include <librd/rdlog.h>

        void f() { rdlog(LOG_ERR, \"TEST\"); }"
...

Other configurations could use installation under local folder with
--prefix or DESTDIR and include that one.

[eugpermar]

Please review it carefully, your knowledge of bash are far more advanced than mine and I'm sure that a lot of stuff can be done in a better way, but I think this PR could be a good base :)

[edenhill]

Great stuff!

[edenhill]

This gets a little noisy, what do you think of adding just a function reference?

function bootstrap_librd {
   mkl_lib_bootstrap $URL $my_ccflags ....
}

Or maybe automatically resolve the bootstrapper so that if mkl_bootstrap__<confname> is available it is called automatically from mkl_lib_check() if configure --bootstrap ?

[eugpermar]

In a first step I though in add them as you said, but then I had problems with recursion, since mkl_lib_check will call itself for bootstrap forever. It needs a break condition, maybe another flag, or internally delete the variable once we have called bootstrap as fallback.

[eugpermar]

Currently it goes via mkl meta bootstrap_fn, but if you prefer it could be with `bootstrap_${config_name}" and let it auto-discovery. We will save one mkl_meta_set that way.

[edenhill]

refactor this into separate functions

[eugpermar]

Hi! I've updated with dependency meta variables. Could you please check the changes?

Thanks!

[eugpermar]

Hi, Magnus! Were you able to check this? I can try the bootstrap_<conf_name> approach if you see it more convenient, but I think that meta variables give more flexibility than that. In the future, they can install dependencies with apt-get or dnf, for example, and stuff like that. ¿What do you think?

Thanks!