Some changes in the new build system

Added a global "run" target as well as individual "run_$name_of_test"
targets for each test. If you use the "run" target, please always call
it with -j1.

For building, make's -j option should better be set using the MAKEFLAGS
environment variable.
Readme: add note about MAKEFLAGS
Merge pull request #7 from Tilka/build_system

Some changes in the new build system