Fixes

The commit history is very bad, I should probably `rebase -i` before merging ...
Maybe we should avoid the  `?ccopt` and `?cclib` arguments (for the latter we should anyway try to specify them as C `Pkg` deps and try to resolve flags through  `pkg-config`). Users can simply go through `?flags` and 
`Flags.{cclib,ccopt}` (to which we should ad an optional `?available` argument). Same for js and `jsflags` below. This makes more clear that there is a single mechanism to specify flags.