Add cmake function to gather cmd arguments

cmake/AwsGetCmdArguments.cmake

@@ -0,0 +1,36 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0.
+
+# Get list of variables provided via command line arguments (i.e. -DVARIABLE=VALUE). This function iterates over all
+# variables and determines if they're provided via command line or not by checking their help strings. CMake sets help
+# strings for variables provided via command line to the "No help, variable specified on the command line." phrase since
+# at least v3.0.
+# Via https://cmake.org/pipermail/cmake/2018-January/067002.html
+#
+# MUST be done before call to 'project'. The reason is that project() resets help strings for some variables
+# (e.g. CMAKE_INSTALL_PREFIX).
+# 
+# Populate AWS_CMAKE_CMD_ARGS with command line variables and their values.
+function(aws_get_cmd_arguments)
+    if (PROJECT_NAME)
+        message(WARNING "aws_get_cmd_arguments is called after project(), some variables may be missed")
+    endif()
+
+    if (AWS_CMAKE_CMD_ARGS)
+        message(STATUS "AWS_CMAKE_CMD_ARGS variable is already set, resetting it")
+        set(AWS_CMAKE_CMD_ARGS "")
+    endif()
+
+    get_cmake_property(vars CACHE_VARIABLES)
+    foreach(var ${vars})
+        get_property(currentHelpString CACHE "${var}" PROPERTY HELPSTRING)
+        if ("${currentHelpString}" MATCHES "No help, variable specified on the command line.")
+            set(escaped_var ${${var}})
+            # To store a list within another list, it needs to be escaped first.
+            string(REPLACE ";" "\\\\;" escaped_var "${${var}}")
+            list(APPEND AWS_CMAKE_CMD_ARGS "-D${var}=${escaped_var}")
+        endif()
+    endforeach()
+    # Store cmd variables in the cache.
+    set(AWS_CMAKE_CMD_ARGS ${AWS_CMAKE_CMD_ARGS} CACHE STRING "Command line variables" FORCE)
+endfunction()