Add script to conveniently build the bundled cabal

e.g.:
  make CABAL=$(./build-cabal.sh)

One can also set CABAL0_ARGS environment variable.

Author: hasufell

build-cabal.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -e
+
+cabal build \
+	${CABAL0_ARGS} \
+	-j \
+	--disable-tests \
+	--project-dir libraries/Cabal \
+	--builddir=$(pwd)/build/stage0 \
+	--ghc-options="-fhide-source-paths" \
+	cabal-install:exe:cabal 1>&2
+
+exec cabal list-bin \
+	${CABAL0_ARGS} \
+	-v0 \
+	-j \
+	--disable-tests \
+	--project-dir libraries/Cabal \
+	--builddir=$(pwd)/build/stage0 \
+	--ghc-options="-fhide-source-paths" \
+	cabal-install:exe:cabal