README: added Dependencies section, updated Compiling and renamed to Building.

dep/README: fully linkified; tidy.
Updated copyright years.
main: diagnostic-ignored -Wold-style-cast; removed broken [[noreturn]].¹

1: Clang 3.5 claims handle_interrupt() is a type (even if the
declaration were removed and [[noreturn] was moved to the definition),
and my GCC (4.7.3) has no clue what I'm talking about.