Rely on evaluation in cmCustomCommandGenerator for the generators.
When tracing target dependencies, depend on the union of dependencies
for all configurations.
Until now the cmCustomCommandGenerator was used only to compute the
command lines of a custom command. Generalize it to get the comment,
working directory, dependencies, and outputs of custom commands. Update
use in all generators to support this.
Evaluate in the COMMAND arguments of custom commands the generator
expression syntax introduced in commit d2e1f2b4 (Introduce "generator
expressions" to add_test, 2009-08-11). These expressions have a syntax
like $<TARGET_FILE:mytarget> and are evaluated during build system
generation. This syntax allows per-configuration target output files to
be referenced in custom command lines.
The Makefile, VS, and Xcode generators previously duplicated some custom
command line generation code. Factor this out into a separate class
cmCustomCommandGenerator shared by all generators.
