Enable CPack

In our project, we use the following local patch:
```
$ cat cpack.diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 83024b94a..1945cc145 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -169,6 +169,13 @@ endif()

add_sdks()

+SET(CPACK_PACKAGE_DESCRIPTION_SUMMARY "AWS C++ SDK")
+SET(CPACK_PACKAGE_VENDOR "Amazon")
+SET(CPACK_PACKAGE_VERSION "${PROJECT_VERSION}")
+SET(CPACK_GENERATOR "RPM;DEB")
+SET(CPACK_DEBIAN_PACKAGE_MAINTAINER "Package fairy")
+INCLUDE(CPack)
+
# for user friendly cmake usage
include(setup_cmake_find_module)
```
After this, I can
`$ cd ; make; cpack -G RPM`
on a RedHat-like system or
`$ cd ; make; cpack -G DEB`
on a Debian-like system, to create an installable package file.

It would be nice to have this by default, with proper values for the CMake variables, of course.