feedback on files being processed by build-tools

Calling "stack build" currently doesn't report the files that are being processed by build-tools such as happy and alex.
When multiple files are processed by build-tools,
it is no longer clear to which file the reported errors and/or warnings belong.

Could the files being processed by build-tools be echoed,
such that it becomes clear to which file a message belongs?

To give an example:
I prefer
```
alex sys/front/TxsAlex.x
alex sys/smt/RegexAlex.x
alex sys/smt/SMTAlex.x
alex sys/smt/SMTStringAlex.x
happy sys/front/TxsHappy.y
unused rules: 4
unused terminals: 3
happy sys/smt/RegexPosixHappy.y
happy sys/smt/RegexSMTHappy.y
shift/reduce conflicts: 15
happy sys/smt/SMTHappy.y
happy sys/smt/SMTStringHappy.y
```

over the current output
```
unused rules: 4
unused terminals: 3
shift/reduce conflicts: 15
```
According to the stack developers (see
https://github.com/commercialhaskell/stack/issues/2843#issuecomment-276249745)
this is a cabal issue!

Start by tracing how Cabal invokes the build-tools mentioned in the issue, especially `happy` and `alex`, and compare their current output with the requested `stack build` example. The work is done when each processed input file is echoed before its tool's warnings or errors, with the existing build behavior preserved.

Written by the indexing model from the issue text.