getBaseDir doesn't work with -fexternal-interpreter
There is some logic in order to work out the base directory relative to the executable path in SysTool.BaseDir but
it doesn't work because getExcutablePath returns the path to the iserv binary and not ghc.
So, using any program which uses the GHC API (such as GHC) will fail to find the files in the right place.
Workaround is to set _GHC_TOP_DIR explicitly to the right value.
At least getBaseDir should fail when using external interpreter as the path it returns is bogus.