(fixes #259) Minimize rebuilds due to git repository updates

Adds a new waf configure option --minimize-version-updates. This option replaces the current version building behavior, which checks for version updates every build, with one that only checks for version updates when version information is missing.

The current behavior of the version code is to check the git repository for changes (new commits, tags etc.) every time waf build is called. If new changes are found, the version headers are updated with the new information and the core module is rebuilt. Depending on ones development process/style, frequent updates to the git repository will cause frequent rebuilds of the core module which may be undesirable.

The goal of the --minimize-version-updates option is to provide version information but decrease the frequency of repository checks and module rebuilds. When the --minimize-version-updates option is used it will disable the current behavior of checking for updates every time waf build is called and instead only perform repository checks when the version information is missing (like after a call to waf clean or changing the output directory).