Add support for cross-compiling win32 executable on Linux
doc: update README and piqi.1 manpage

In addition:

- when generating the manpage, set its date based on the date of the actual
  latest change in git
- don't update manpage automatically during "make doc" as it may not always run
  from a cloned git repo
doc: bring documentation up to date
piqi-0.6.6 release
Bump version number to 0.6.6-dev
tests shouldn't fail on missing protoc