configure: detect ICU via icu-uc/icu-i18n pkg-config modules to easily build on Debian

[hugo-hur]

icu-config was deprecated and removed in ICU 63+, so it is absent on modern Debian/Ubuntu where only libicu-dev with pkg-config support is shipped. The pkg-config fallback used the monolithic "icu" module name, which those distros no longer provide, so configure failed to find ICU even when it was installed.

Try the split modules "icu-uc" and "icu-i18n" (used since ICU 58) first, falling back to the legacy "icu" module for older systems. Also fall back to "pkg-config --modversion icu-uc" for the --enable-icu-6x version check when icu-config is unavailable.

The icu-config path is still tried first, so older distros that still provide it continue to work unchanged.

Summary of changes

This pull request includes following changes or fixes.

• Fixed configure having problems finding icu libraries automatically

Description

No other dependencies affected. Just the icu library search on build.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have confirmed my fix is effective or that my feature works

[hugo-hur]

After this change the build is as easy as

apt install libicu-dev libxml2-dev uuid-dev libfuse-dev fuse build-essential automake libtool pkg-config
git submodule update --init --recursive
./autogen.sh
./configure
make

on Debian Trixie

[Piloalucard]

LGTM

[XV02]

Great fix, thanks for working on this since it solves the root cause that made a dummy icu-config necessary. With this change a cleanup follow up is needed in the documentation and some workflows to maintain the codebase consistent with it.

The README.md on the root folders contains references to the old icu-config workaround on lines 292, 294 and 300 that should be removed.

Also the sudo cp instructions for copying the custom icu-config workaround should be removed from both build.sh and codeql-analysis.yml files since they're no longer needed.

Would appreciate your help with the cleanup.