The ar command used to build GHC gets leaked in the built program:

# ghc --info | grep 'ar command'
 ,("ar command","/home/pbulk/build/lang/ghc7/work/.wrapper/bin/ar")

Should be:

# ghc --info | grep 'ar command'
 ,("ar command","ar")

I can't find how these are being created. The only workaround I can think of is putting /home/pbulk/build/lang/ghc7/work/.wrapper/bin/ in the PATH when building GHC and configuring it with --with-ar=ar.