Add a ./configure flag "DIST_LANG".

The flag can be used to select which language to include in the distribution package.

For example:
  $ ./configure && make dist  // include protoc and C++ runtime only.
  $ ./configure DIST_LANG=cpp && make dist  // the same as above.
  $ ./configure DIST_LANG=java && make dist  // include protoc, C++ runtime and Java runtime.
  $ ./configure DIST_LANG=all && make dist  // include protoc and the runtime for all languages.

Fixes issue #107 

@pherl