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.

CMake: add CMakeForegroundComplexCommands option

When this option is set, custom_commands that have been passed
USES_TERMINAL_DEBUG will get USES_TERMINAL set which puts them in the
console pool in Ninja. This results in direct access to stdio which
allows for direct logging of build output or the possibility of user
input.

cmake: New CakeML version replaces arm6 with arm7

ARMv6 is no longer a supported target and ARMv7 replaces it.

CMake: Add support for multiple CakeML libraries

Holmake allegedly doesn't support being called to concurrently build
multiple targets with common dependencies. We need to serialise
invocations of Holmake and achieve this by forcing each CakeML library
to depend on the previously defined CakeML library. These are order-only
dependency constraints and so if one library is stale it won't force
unrelated libraries to be rebuilt unnecessarily.

CMake: Construct CakeML build files from templates

The behavior of DeclareCakeMLLib is unchanged, but it should be easier
to maintain the build files without them being inlined in a CMake file.

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.

Remove bad project definition in cakeml.cmake

This shouldn't be here as it is a helper/module and not a project.

style: style cmake files consistently

cakeml.cmake: Parameterised Cake Binary

Extended cakeml.cmake to find the cake binary. This in addition
allows the user to pass in the Cake binary from command line. This
is required if the user wishes to use a specific cake binary that
targets a different architecture.

cakeml: update for meta_utils usage

1. Make the Holmake build step depend on the dependencies passed to
DeclareCakeMLLib
2. Pass CAKEMLDIR as an environment variable when invoking Holmake so that
it can be picked up by child Holmake processes

CMake: ARMV6-8 support for CAKE_TARGET

Infer the CAKE_TARGET for ARMV6-8 platforms.

cakeml: fix list syntax for multiple HOL includes

cakeml: pass extra options to Holmake

It is now possible to run a build with:

../init-build.sh -DHOL_EXTRA_OPTS="--fast;--qof"

Arguments must be semicolon separated (i.e. a CMake list)

cakeml: update buildScript.sml template for new CakeML version

cmake-tool: Allow rm'ing zero files

Passing -f the rm handles the freshly initialize case where there are not yet any files
to remove and so rm would complain about missing operands.

cmake-tool: Remove all symlinked .sml files

This removes any file in the CakeML build directory that is both a symlink and a .sml file
instead of just Script.sml files. Unfortunately the user might have provided additional .sml
files that are not Script.sml, and HOL will spew intermediate files with the .sml extension.
The easiest way to distinguish is to check which ones are symlinks.

cmake: fix CakeML buildScript.sml template

Holmake was having issues parsing the dependencies from the `open` statement
sans semicolon.

cmake: Symlink CakeML sources instead of copying

Symlinking the CakeML sources into a dedicated directory allows for treating this directory
as a HOL development directory to iterate the sources without having to remember to replay
any changes to the 'original' files prior to rebuilding and recopying the files.

This change has no functional differences, it is purely intended to make life easier for
developers who are actuall writing and iterating on CakeML/HOL code in.

trivial: tabs->spaces

cmake-tool: Support generator expressions in CakeML sources

Changes the copying of CakeML sources to have a single copy command that produces a stamp
file. This allows for the source files list to contain generator expression lists.

cmake: Helper for compiling cakeML HOL theories

Provides a helper function for creating a linkable library out of some cakeML HOL theories