Make pkg-config dependency more robust

[andreasabel]

Lifted from haskell/cabal#8496 (comment). @gbaz writes:

I note btw that text-icu with its current cabal file will always fail if there is no pkg-config present on the system, or if icu-uc, or icu-i18n are not registered in it.

It would be better to have an auto flag that switches between a build using pkgconfig-depends and a build explicitly enumerating extra-libraries. This is what the pr that started this all was intended to enable :-) haskell/cabal#7621

In such a case if the CI busted on pkgconfig-depends, then it will fallback to the explicit extra-libraries etc.

This also makes the package somewhat more resilient for end-users.

My 2 cents to this: One would have to investigate whether such an auto flag also works with stack which has a different flag semantics.