Add test for custom command with a genex referring to a target.

This test failed before d8a59ea4 (Port cmGeneratorExpression to
cmTarget from cmGeneratorTarget, 2012-10-27), and passes after.
Merge topic 'test-genex-custom-command' into next

08bd86b Add test for custom command with a genex referring to a target.
Don't crash when a target is expected but is not available.
Merge topic 'fix-genex-with-no-target' into next

e767ffc Don't crash when a target is expected but is not available.