There were a few unused LDFLAGS and LD_LIBRARY_PATH settings. There was
also an incorrect setting; Julia prefers to be linked to threaded
FFTW. The libary names are now set correctly so that it's not necessary
to link system libraries into $out/lib/julia as if they were private
libraries. The LDFLAGS were also moved out of NIX_LDFLAGS so they will
affect only the Julia build itself, and not all the individual private
libraries.