Add --disable-llvm-backend/--enable-llvm-backend build-time options, and move llvm.c to src/backends/llvm/ subdir

Scons now doesn't throw away /home/stewart/.nix-profile/lib/pkgconfig:/nix/var/nix/profiles/default/lib/pkgconfig:/run/current-system/sw/lib/pkgconfig, so NixOS can build it

Wrapping the SHLINKFLAGS in an array rather than as a single string
caused the link flags to be passed to clang double quoted. This cuased
clang to interpret the flags as a string rather than as a flag which
caused the -install_name flag to never be passed to the linker.