Should not create empty directories in setup install/copy

[bos]

(Imported from Trac #97, reported by @dcoutts on 2006-10-31)

When installing a library, in this case MissingH 0.16, I get the error:

ghc-pkg: /opt/local/lib/MissingH-0.16.0/ghc-6.6/include doesn't exist or isn't a directory (use --force to override)
Error: Target com.apple.activate returned: shell command "/opt/local/libexec/missingh/register.sh" returned error 1
Command output: Reading package info from stdin ... done.
ghc-pkg: /opt/local/lib/MissingH-0.16.0/ghc-6.6/include doesn't exist or isn't a directory (use --force to override)
Warning: the following items did not execute (for missingh):
com.apple.activate
Error: Status 1 encountered during processing.

(the above is the output of the `MacPorts?` portfile.)

The problem is that when the build is staged into a temporary
location so the files can be inventoried, an empty directory,
<prefix>/lib/MissingH-0.16.0/ghc-6.6/include is created. MacPorts?
deletes this empty directory (along with a number of other unused
directories it creates in the staging process). The registration
process requires the empty directory to be present and fails, as shown above, if it is not.

[bos]

(Imported comment by ross on 2006-10-31)

It is untidy of Cabal to create an unnecessary directory, but it is clearly a bug for MacPorts? to remove a directory it did not create.

[bos]

(Imported comment by @dcoutts on 2007-02-25)

The include dir will now only be made and noted in the package registration info if we actually install some include files.