Prepare for 1.5 release.
build: In MSVC builds, use 'rc' instead of 'windres'.
Improve copyright statement in the Windows .rc file.
Update to Unicode version 18.0.0.
INSTALL.windows: Fix MSVC instructions.
build: Fix "make installcheck" failure in --disable-shared builds.
Add an LLM policy.
Upgrade to libtool 2.6.2.