Summary:
When we introduced user-friendly library names
(e.g. unix-2.7.1.0-G4Yo1pNtYrk8nCq1cx8P9d instead of
unix_G4Yo1pNtYrk8nCq1cx8P9d) we added a new variable to
be written out by ghc-cabal, $1_$2_LIB_NAME.

What I didn't realize at the time was that this conflicts
with an existing variable in the build system, $1_$2_$3_LIB_NAME,
which (confusingly) refers to something like
'libHSunix-2.7.1.0-G4Yo1pNtYrk8nCq1cx8P9d.so'.  This is pretty
confusing (despite never conflicting), so I renamed this variable
to LIB_FILE for enhanced greppability.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1002