Allow building a subset of individual targets defined in a build script

[lewissbaker]

Sometimes you will have a build script that builds an entire library potentially consisting of many source files. In addition it might also be a shared library (DLL) that requires linking with other libraries built in other build scripts. In the case of a top-level program build script it might end up building every source file.

Currently if you ask Cake to build a build script it builds everything in that build script and recursively builds everything required to build those things.

What would be nice is having a way to specify that you only want to build a certain target within a build script rather than having it implicitly build everything in that script. eg. only build an individual source file within a library or build all object files but skip any link step that might require building other libraries.

Imagine extending the command-line to allow specifying an optional list of target names after each build script. eg.

$ cake src/build.cake:foo.obj    # Just build foo.obj
$ cake src/build.cake:foo.obj,bar.obj,baz.obj    # Build multiple named targets

Build scripts (or tools called by build scripts) could register named targets with the current root script using an API that looks something like the following:

from cake.tools import script, sometool

# Tool may automatically register the returned target under the name 'sometarget'.
someTarget = sometool.build(target='sometarget')

# Script could manually register the target under a different name
script.addTarget('anothername', someTarget)

Build scripts could refer to named targets from other build scripts to indicate some dependency or to republish the other build script's targets under a name in the current build script.

from cake.tools import script

otherScript = script.get(script.cwd('other.cake')

# Get a reference to a named target from another script.
# Note this would not cause other.cake to be executed unless the returned target
# was used by something.
otherObjects = otherScript.getTarget('objects')

# Could republish the other script's named targets as our own so that when
# this script is asked to build 'objects' it would result in building 'objects'
# target from other.cake.
script.addTarget('objects', otherObjects)

In terms of implementation, we'd need to introduce the concept of a lazy task or lazy target so that a script can execute, define how to build a bunch of targets but not actually build any of them unless those targets have been requested to be built.

If the user just asks for a build script to be built without specifying a named target (as is currently supported) then the behaviour should be as per the current behaviour. ie. build all targets defined in that build script.

[lewissbaker]

Some groundwork towards this has been added in 2b8580d which added support for lazy-tasks.

Main outstanding work is updating the command-line arg parsing to support specifying named targets of a script from the command-line. eg. to allow something like "cake path/to/build.cake@target1,target2"

[lewissbaker]

Support for specifying named targets on command-line is implented in 1240b6d

Also added -l and --list-targets option for listing named targets defined in the scripts specified on command-line.