Support u-config as an alternative of pkg-config

[bombless]

An implementation of pkg-config, u-config, doesn't support --list-all option, so it would be nice if Cabal can work without it.
The status quo is that Cabal will decide pkg-config is unavailable if pkg-config --list-all report failure.
See also skeeto/u-config#7

Is there any reason Cabal don't use pkg-config --modversion directly to query desired libraries?