cmTarget: Make the source files depend on the config.

Disallow the use of config-specific source files with
the Visual Studio and Xcode generators. They don't have
any way to represent the condition currently.

Use the same common-config API in cmQtAutoGenerators. While
it accepts config-specific files, it doesn't have to support
multiple configurations yet.

Loop over the configs in cmTargetTraceDependencies
and cmGlobalGenerator::WriteSummary and consume all source
files.

Loop over the configs in cmComputeTargetDepends and compute the
object library dependencies for each config.

Tests/ConfigSources/CMakeLists.txt

@@ -0,0 +1,9 @@
+
+cmake_minimum_required(VERSION 3.0)
+
+project(ConfigSources)
+
+add_executable(ConfigSources
+  $<$<CONFIG:Debug>:main.cpp>
+  $<$<CONFIG:Release>:does_not_exist.cpp>
+)

Tests/ConfigSources/main.cpp

@@ -0,0 +1,5 @@
+
+int main(int argc, char** argv)
+{
+  return 0;
+}