If a subproject is not required and fails during its configuration, the
parent project continues, but should not include any target or state set
by the failed subproject. This fix ninja still trying to build targets
generated by subprojects before they fail in their configuration.
The 'build' object is now per-interpreter instead of being global. Once
a subproject interpreter succeed, values from its 'build' object are
merged back into its parent 'build' object.
Xavier Claessens