Summary
The CMakeLists.txt currently downloads and installs the AWS SDK into the build folder, using ExternalProject_Add, which requires a network connection, git to be installed, and a significant amount of build time. There should be an option to use a local version of the SDK, so that it doesn't have to be redownloaded and reinstalled, if the user already has it installed on their machine.

Suggested Solution
Use a config_choice variable so that the user can choose to either:

1. Use a system installed AWS SDK, via find_package (TODO)
2. Fetch and build the project via an ExternalProject_Add (currently the only behaviour)

I.e.

config_choice(    
    IMPORT_S3_SDK    
    "Specifies how to import the S3 SDK"    
    OPTIONS    
    "none;IMPORT_S3_SDK_NONE;"    
    "package;IMPORT_S3_SDK_PACKAGE;HAVE_S3_SDK_PACKAGE"    
    "external;IMPORT_S3_SDK_EXTERNAL;"    
)    

then have an if-else depending on the config value (generalized):

if(IMPORT_S3_SDK_NONE)
   return()
elseif(IMPORT_S3_SDK_PACKAGE)
   find_package(AWSSDK)
   ....
else()
   ExternalProject_Add(...)
   ...
endif()