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As apparently there is already a test named T5863,
I named this one T5863a.

(Thanks to Martijn for correcting this oversight.)

We used to use the base package to do the detection. However, base has
    extra-lib-dirs: @ICONV_LIB_DIRS@
in base.buildinfo.in, which means that if ICONV_LIB_DIRS is set then
the package config will have
    library-dirs: [...]base/dist-install/build /path/for/iconv
and, as paths may be double quoted and contains spaces, this makes it
a little tricky to extra the path that we want to look in (the first
path).

We therefore now use the ghc-prim package, which has no such complication.

We were checking paths with
    if [ -f '"/path/to/Prelude"' ]
i.e. the " quotes were being quoted by the ' quotes. Now we just
use " quotes (which come from the ghc-pkg output).