convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

make.cmake: Use printf instead of echo

This removes the need to try and escape the newline at the end of the
echo and simplifies these commands.

cmake-tool,make: add create_depfile_by_find macro

This macro creates a list of options to append to an add_custom_command
call arguments that construct a GNU make style dependency file based on
all the files enumerated in a given source directory. It is intended to
be used in situations where another build script is invoked during the
build process where the dependencies for the script are contained within
a directory structure. This process is faster than using CMake's
GLOB_RECURSE file call and is handled by Ninja. It will only work on
Ninja generation targets.

CMake: Refactor helpers to be included on demand

As more helper files get added it becomes more expensive to include them
all in common.cmake. We instead add them to the CMAKE_MODULE_PATH and
allow them to be imported as CMake modules. Adding global include guards
allows them to be imported multiple times without processing every time.

cmake: Add make helpers

MakefileDepsToList: Parses a make dependes depfile, which is a list of
space separated dependencies after a colon separated output file, and
returns a cmake list of dependencies.

execute_process_with_stale_check: Wraps a call to execute_process with
checks that only rerun execute_process if the command or input files are
changed. It also uses a depfile to track any files that the command
touches internally. This function currently won't work without a
depfile.