Change how we detect what library ways are available

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.