Honour previous values for CABAL and CABFLAGS

The immediate goal is let the hie-bios.bat script
set CABFLAGS with `-v0` and remove all cabal output
except the compiler arguments

hadrian/build-cabal.bat

 @echo off
-set CABAL=cabal
-set CABFLAGS=--disable-documentation --disable-profiling --disable-library-profiling
+
+if "%CABAL%"=="" (
+    set CABAL=cabal
+)
+
+if "%CABFLAGS%"=="" (
+    set CABFLAGS=--disable-documentation --disable-profiling --disable-library-profiling
+)
 
 rem It is currently more robust to pass Cabal an absolute path to the project file.
 set PROJ="%CD%/hadrian/cabal.project"