Add aws_get_cmd_arguments_for_prebuild_dependency

cmake/AwsPrebuildDependency.cmake

@@ -75,3 +75,39 @@ function(aws_prebuild_dependency)
         DESTINATION ${CMAKE_INSTALL_PREFIX}
     )
 endfunction()
+
+# Get list of variables provided via command line arguments (i.e. passed as -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
+#
+# NOTE The project() call 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_for_prebuild_dependency)
+    if (AWS_CMAKE_CMD_ARGS)
+        message(DEBUG "AWS_CMAKE_CMD_ARGS variable is already set, resetting it")
+        set(AWS_CMAKE_CMD_ARGS "")
+    endif()
+
+    set(variables_to_ignore CMAKE_INSTALL_PREFIX)
+
+    get_cmake_property(vars CACHE_VARIABLES)
+    foreach(var ${vars})
+        get_property(currentHelpString CACHE "${var}" PROPERTY HELPSTRING)
+        if ("${currentHelpString}" MATCHES "No help, variable specified on the command line.")
+            if("${var}" IN_LIST variables_to_ignore)
+                # TODO Remove.
+                cmake(WARNING "Ignoring ${var}")
+                continue()
+            endif()
+            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()