Allow libicu to be, via pkg-config, installed in a custom path

An attempt towards a more correct fix for !3385 (closed)

Samba configuration uses pkg-config to locate libicu, and then probes for its constituent libraries (icui18n, icuuc, icudata). This succeeds if libicu is installed as a system library, otherwise CHECK_LIB fails because the -I and -L options provided by pkg-config are not used.

This reworks our pkg-config handling so this all works in a much more natural fashion.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15623

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code