Revert "configure.ac: honour LLVM_LIBDIR when linking against LLVM"

Revert "configure.ac: honour LLVM_LIBDIR when linking against LLVM"

This reverts commit a39ad185932eab4f25a0cb2b112c10d8700ef242.

The commit aims to address "missing" -L/foo/bar during linking stage. At
the same time it doesn't add the -L and yet the LLVM_LDFLAGS [which
provide -L/foo/bar] are already used throughout.

Seems like something pretty unique (broken?) on my end. Since the commit
introduces issues (due to the missing -L) revert until we get to the
root of it (PEBKAC or a genuine issue).