Build windows against icu

[h-vetinari]

Trying to make the build more uniform, see #168, following instructions from upstream (doesn't appear to have the same b2-interface like the other libs).

Plus some minor clean-ups (which I avoided for unification to keep the diff minimal).

Closes #168

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

This seems to be running into something vaguely similar to bfgroup/b2#189, as we're getting:

    - has_icu builds           : no [2]
    - iconv (libc)             : no [2]
    - iconv (separate)         : yes [2]
    - icu                      : no [2]

[h-vetinari]

Still nada:

    - icu                      : no [2]
    - iconv (libc)             : no [2]
    - iconv (separate)         : yes [2]

[sdebionne]

Maybe it is worth to try to set ICU_PATH to %LIBRARY_PREFIX% ?

[h-vetinari]

Maybe it is worth to try to set ICU_PATH to %LIBRARY_PREFIX% ?

already done

[h-vetinari]

No change.