Expose libexec option in simple build system

(Imported from [Trac #717](http://hackage.haskell.org/trac/hackage/ticket/717), reported by guest on 2010-07-23)

For my Bluetile Cabal package (
[http://hackage.haskell.org/package/bluetile](http://hackage.haskell.org/package/bluetile) ) I would like to install some of the executables into the libexec target directory.

Cabal already supports a libexec directory, there is just no easy way to get an executable copied there. So I had to write my own custom hooks.

To simplify things, I would like to suggest the option of marking an executable to be installed in the libexec directory. I propose a simple flag 'Is-Helper-Executable: True/False' which could be specified as part of an 'Executable' section. Setting this to True should result in Cabal installing the executable to the libexec directory.

Quick example of why one wants to use the libexec directory in the first place: Bluetile has a number of such helper applications. While having Bluetile packaged for Debian, it turned out that Debian requires a man page for every binary under /usr/bin. So I would have had to write man pages for every helper application or - which seemed like the proper way to do it - have them installed under /usr/lib/bluetile/ (libexec directory).