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 or
bindir, so the object files and executable will be within the tree.
Outsource test.c

The test.c file from which the test executable "tm" is created gets its own
directory, so that more (dedicated) tests can be added without clobbering
the actual library source code.
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.