Cabal cannot find GHC when using relative path in -w flag

(Imported from [Trac #768](http://hackage.haskell.org/trac/hackage/ticket/768), reported by @tibbe on 2010-11-19)

Trying to build the network package while standing at the root of a GHC build tree fails to find GHC:

$ cabal install -w inplace/bin/ghc-stage2 network -v2

inplace/bin/ghc-stage2 --numeric-version

looking for package tool: ghc-pkg near compiler in inplace/bin

found package tool in inplace/bin/ghc-pkg

inplace/bin/ghc-pkg --version

inplace/bin/ghc-stage2 --supported-languages

Reading installed packages...

inplace/bin/ghc-pkg dump --global

inplace/bin/ghc-pkg dump --user

inplace/bin/ghc-stage2 --print-libdir

Reading available packages...

Resolving dependencies...

selecting network-2.3 (hackage) and discarding network-2.0, 2.1.0.0, 2.2.0.0,

2.2.0.1, 2.2.1, 2.2.1.1, 2.2.1.2, 2.2.1.3, 2.2.1.4, 2.2.1.5, 2.2.1.6, 2.2.1.7,

2.2.1.8, 2.2.1.9, 2.2.1.10, 2.2.3 and 2.2.3.1

selecting base-4.3.0.0 (installed)

selecting ffi-1.0 (installed)

selecting ghc-prim-0.2.0.0 (installed)

selecting integer-gmp-0.2.0.2 (installed)

selecting rts-1.0 (installed)

selecting parsec-2.1.0.1 (hackage) and discarding parsec-2.0, 2.1.0.0, 3.0.0,

3.0.1 and 3.1.0

selecting unix-2.4.1.0 (installed) and discarding unix-2.0, 2.2.0.0, 2.3.0.0,

2.3.1.0, 2.3.2.0, 2.4.0.0, 2.4.0.1 and 2.4.0.2

selecting bytestring-0.9.1.8 (installed) and discarding bytestring-0.9,

0.9.0.1, 0.9.0.2, 0.9.0.3, 0.9.0.4, 0.9.1.0, 0.9.1.1, 0.9.1.2, 0.9.1.3,

0.9.1.4, 0.9.1.5, 0.9.1.6 and 0.9.1.7

In order, the following would be installed:

parsec-2.1.0.1 (new package)

network-2.3 (new package)

parsec-2.1.0.1 has already been downloaded.

Extracting

/home/tibell/.cabal/packages/hackage.haskell.org/parsec/2.1.0.1/parsec-2.1.0.1.tar.gz

to /tmp/parsec-2.1.0.117969...

Configuring parsec-2.1.0.1...

cabal: Cannot find the program 'ghc' at 'inplace/bin/ghc-stage2' or on the

path

cabal: Error: some packages failed to install:

network-2.3 depends on parsec-2.1.0.1 which failed to install.

parsec-2.1.0.1 failed during the configure step. The exception was:

ExitFailure 1