Move all logic for headers/libraries for each shared library
into the shared library scope - similar to how OpenSSL already works.

Also, don't assume that b() returns a boolean.

Attempt to fetch information from pkg-config if available.
Also, let the user provide a custom PKG_CONFIG path, based on a
floating patch by Paul McClave <pmcclave@chromium.org>.

Additionally, make library input parsing more robust as well as
deduplicating what we provide to the linker.

Minor edits to current build flags and its help texts as well
as grouping shared and i18n options into separate option groups.

Hopefully this will improve the readability of `./configure` output.

Avoid custom validation as well as simplify output generated
by configure. Add 'system-icu' to the list of valid choices.