Require system lzma and libarchive

[pinotree]

• require lzma and libarchive as system libraries, dropping their local download by default; they are available as standard libraries in Linux distros (and also in other Unix systems)
• require some tools (autotools, patch, etc) only when building squashfuse ourselves
• add a USE_SYSTEM_XDGUTILS for consistency with other ExternalProject's

[TheAssassin]

If you really want to remove the local build options, you either need to make sure these libraries are available and recent enough in https://github.com/AppImageCommunity/AppImageBuild, our build environment.

The modified project no longer links the two libraries in question statically, which is likely to cause issues. Unfortunately, CMake's built-in pkg-config support doesn't support imported targets for static libraries as of yet, and neither does our own function.

[pinotree]

If you really want to remove the local build options, you either need to make sure these libraries are available and recent enough in https://github.com/AppImageCommunity/AppImageBuild, our build environment.

Hmm I saw that ci/install-deps.sh already had the system packages for them and assumed it was enough - OK, will take a look at AppImageBuild.

The modified project no longer links the two libraries in question statically, which is likely to cause issues.

Can you please expand on the reasons why this would be problematic? Usually you want to dynamically link to this kind of system libraries, as it makes less problematic to update users of them in case of e.g. bugfixes/CVEs/etc.