Add SPDX license tags to build system files

[robn]

Motivation and Context

#18074 prompted me to look into exactly what the deal is with licensing around autoconf extensions/macros (tl;dr: no big deal). Since I was in there thinking about licenses anyway, I went through and added the tags

Closes #18074.

Description

Build files are *.am, *.ac and *.m4. Almost all are CDDL-1.0, as you'd expect.

Most third-party macros are from the autoconf archive, and are under FSF "Permissive" or "Unlimited" licenses (FSFAP and FSFULLR).

The other two are regular GPL licenses with "exceptions". The SPDX identifier format actually has this whole grammar for expressing combinations and add-ons, but we don't need most of it, so I've just hardcoded those two as specific long strings in spdxcheck.pl.

How Has This Been Tested?

Compile checked on Linux and FreeBSD for completeness, but since it's entirely adding comments I wouldn't expect anything.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.