Patch: Add support for stable build dir name (subst pattern support)

[abierbaum@…]

This is a patch that I put together to help me support a build where I want to use the same build directory over and over. The reason I needed this is that 95% of the compile time for the build is to compile external dependencies that rarely change. By using the same directory and keeping the files, I am able to reuse the partial builds from the previous run.

This change introduces a few small but important changes:

• The build master now sends the Configuration name and build id over to the build slave to be used as context vars. These are named 'config_name' and 'build_id' respectively. (see bitten/master.py)
• Fix a bug related to canceling the build when using the dry run flag. (see ticket: 187)
• Add a new command line option '--build-sub-dir' to the slave application and the Slave object. This variable is used to specify the name to use for the subdirectory within the build. It can use context variables for substitution. (see below)
• Reorder the logic in slave _execute_build to initialize the Recipe, Context, and create the basedir based on the new capabilities.
• When setting the basedir for a context, use config.interpolate to allow the name to be formed from context vars (uses 'build_${build_id}' by default)

Note: This patch captures the fix from ticket:187 as well.

[anonymous]

The list of changes should read: (sorry about that)

• The build master now sends the Configuration name and build id over to the build slave to be used as context vars. These are named 'config_name' and 'build_id' respectively. (see bitten/master.py)
  
• Fix a bug related to canceling the build when using the dry run flag. (see ticket: 187)
  
• Add a new command line option '--build-sub-dir' to the slave application and the Slave object. This variable is used to specify the name to use for the subdirectory within the build. It can use context variables for substitution. (see below)
  
• Reorder the logic in slave _execute_build to initialize the Recipe, Context, and create the basedir based on the new capabilities.
  
• When setting the basedir for a context, use config.interpolate to allow the name to be formed from context vars (uses 'build_${build_id}' by default)
  

[cmlenz]

(fix description formatting)

[cmlenz]

Patch applied in [513], though slightly modified (the names are shorter, basically).

Many thanks!