Prefix calcN example executables with x3_ to avoid error.

At least on Linux, b2 fails to link calc{7,8,9} since the exe name is identical
to the subdirectory containing the source files.

Prefix all calc examples with x3_ to actually allow to link them.
Merge pull request #71 from mlang/x3/examples/link

Prefix calcN example executables with x3_ to avoid error.