cpack: Add CPACK_INSTALL_CMAKE_CONFIGURATIONS variable

For the multi-configuration generators one can specify the list
of configurations to include in the package.

E.g. having a project, where debug libraries have a suffix to
distinct them from the release builds, one can build the package
containing both `Debug` and `Release` binaries.

Help/release/dev/cpack-install-multiple-configurations.rst

@@ -0,0 +1,5 @@
+cpack-install-multiple-configurations
+-------------------------------------
+
+* CPack learned :variable:`CPACK_INSTALL_CMAKE_CONFIGURATIONS` to control
+  what configurations going to be packaged for multi-configuration generators.