some simple changes to the build

These commit modify the build system by doing the following:

- get CFLAGS and LDFLAGS using pkg_config. Which allows the build to find the libraries in unusual place. (for example with homebrew)
- add a Makefile and remove the shell scripts
- rename README.txt and LICENSE.txt in README.md and LICENSE (so github can parse the content of the README).

Hope it will helps you.

- benoit