Added --with-llvm=<path>

This cleans up needing to set CFLAGS and LDFLAGS externally
and also allows us to call llvm-config to get e.g. the list
of libs (--libs).

Also adds using llvm-config --libs to get the list of libs
to link to for static builds, so static builds work again.

Added an entry to TODO about SDK headers.

Refreshed patches/cctools/809