CMake + Current Playground

In src/:

• hw0.cc: a simple "Hello, World!"
• hw1.cc: depends on a header file, rebuilds when the header file changes.
• hw2.cc: uses dflags and greets --name.
• hw3.cc: needs current_build.h, generated by a special cmake target.
• hw4.cc: links against a library, which is built from its own source file.