Fixed IO exception when attempting to hash a file that no longer exists during incremental tests
Added integration test
More spec updates
More integration test coverage for 'api' library dependencies
Don't create tasks for empty source sets
Remove stale linker outputs when all source files removed
Handle header-only libraries
- static and shared linkages do not have output files with no source files
- removed the use of 'api' linkage from tests where not required.
Unit test demonstrating IOException in incremental compile when header file renamed.
Merge pull request #221 from mputters/incrementalfilenotfound
Fixed integration test so that it works for C and C++