Fix propagation of generator expressions from parent directory

[NWilson]

The CMake build would fail if invoked from a parent directory with a generator expression set on COMPILE_DEFINITIONS:

cmake_minimum_required(VERSION 3.15 FATAL_ERROR)

project(issue-813)

add_compile_definitions($<$<CONFIG:Debug>:DEBUG>)
add_subdirectory(pcre2)

This is a regression.

My code was overly broad; I propagated much more than I intended.

[sergio-nsk]

I would replace the foreach with list(FIND _pcre2_compile_definitions "_GNU_SOURCE" _idx).

[NWilson]

Thank you for reviewing. I have also added a CI test so we do not regress again on this particular behaviour.