Added LibcWrapGenerator.vala and a README file.

The LibcWrapGenerator program generates a header file allowing
one to target older versions of glibc than the version they have
installed on their GNU/Linux system.

The README file explains how to safely use the generator
and the generated header file, and also explains how the
generator and header file work.
Merge pull request #2 from tristanvb/libc-wrap-generator

Added LibcWrapGenerator.vala and a README file.