Always build llvm with llvm

this requires you to have a basic version of llvm in the path to bootstrap

Done because llvm trunk no longer compiles with gcc 4.6
Bugfix in C reference code for a test
Revert to gcc for building llvm trunk

but set LD_LIBRARY_PATH to allow custom installs of gcc
and rejigger the Makefile to respect existing LD_LIBRARY_PATH settings
Storage flattening now legalizes types

Forces elements to be a multiple of 8-bits, and
inserts casts as needed. Fixes an outstanding bug
with vectorized loads of compute_root bool buffers.
Merge branch 'master' into rvar_reorder