Use the Makefile.build project to build

This commit adds the Makefile.build submodule and changes the build process
to use it. Note that Makefile.build currently does not support a builddir,
bindir or libdir, so the object files and executable will be within the tree.
Beautify includes

The CPPFLAGS in the root Makefile add a proper system include path to the
preprocessor (targetting the include/ directory). Using a symlink "tm"
under include/ we can easily namespace our headers, so that

  #include "../include/tm.h"

becomes

  #include <tm/tm.h>

Update all files accordingly.